David Kabat

Protein synthesis during chicken erythrocyte differentiation. Studies of ordered DNA protein fibers (2002)

Kabat, David

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Part I: Protein Synthesis During Chicken Erthrocytes Differentiation. It was the...

The role of IL-2 interaction with p75 and p55 receptor molecules in the stimulation of cell proliferation (1993)

Perkins, Gordon R., Flemming, Claudia L., Kabat, David, Collins, Mary K. L., Simpson, E.

This report addresses the role of individual IL-2 binding proteins of the IL-2 receptor In the stimulation of cell proliferation by IL-2. Murine IL-3-dependent cell lines were established which...

Protein synthesis during chicken erythrocyte differentiation. Studies of ordered DNA protein fibers (1967)

Kabat, David

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Part I: Protein Synthesis During Chicken Erthrocytes Differentiation. It was the...

Cloning and characterization of a cell surface receptor for xenotropic and polytropic murine leukemia viruses

Tailor, Chetankumar S., Nouri, Ali, Lee, Chun G., Kozak, Christine, Kabat, David

Xenotropic and polytropic murine leukemia viruses (X-MLVs and P-MLVs) cross-interfere to various extents in non-mouse species and in wild Asian mice, suggesting that they might use a common receptor...

gp120 envelope glycoproteins of human immunodeficiency viruses competitively antagonize signaling by coreceptors CXCR4 and CCR5

Madani, Navid, Kozak, Susan L., Kavanaugh, Michael P., Kabat, David

Signal transductions by the dual-function CXCR4 and CCR5 chemokine receptors/HIV type 1 (HIV-1) coreceptors were electrophysiologically monitored in Xenopus laevis oocytes that also coexpressed the...

A Sodium-Dependent Neutral-Amino-Acid Transporter Mediates Infections of Feline and Baboon Endogenous Retroviruses and Simian Type D Retroviruses

Tailor, Chetankumar S., Nouri, Ali, Zhao, Yuan, Takeuchi, Yasuhiro, Kabat, David

The type D simian retroviruses cause immunosuppression in macaques and have been reported as a presumptive opportunistic infection in a patient with AIDS. Previous evidence based on viral...

Origin and Rapid Evolution of a Novel Murine Erythroleukemia Virus of the Spleen Focus-Forming Virus Family

Hoatlin, Maureen E., Gomez-Lucia, Esperanza, Lilly, Frank, Beckstead, Jay H., Kabat, David

The Friend spleen focus-forming virus (SFFV) env gene encodes a glycoprotein with apparent Mr of 55,000 that binds to erythropoietin receptors (EpoR) to stimulate erythroblastosis. A retroviral...

An Array of Novel Murine Spleen Focus-Forming Viruses That Activate the Erythropoietin Receptor

Gomez-Lucia, Esperanza, Zhi, Yu, Nabavi, Melud, Zhang, Weibin, Kabat, David, Hoatlin, Maureen E.

The Friend spleen focus-forming virus (SFFV) env gene encodes a 409-amino-acid glycoprotein with an apparent Mr of 55,000 (gp55) that binds to erythropoietin receptors (EpoR) to stimulate...

Effects of CCR5 and CD4 Cell Surface Concentrations on Infections by Macrophagetropic Isolates of Human Immunodeficiency Virus Type 1

Platt, Emily J., Wehrly, Kathy, Kuhmann, Shawn E., Chesebro, Bruce, Kabat, David

It has been proposed that changes in cell surface concentrations of coreceptors may control infections by human immunodeficiency virus type 1 (HIV-1), but the mechanisms of coreceptor function and...

An Endogenous Inhibitor of Human Immunodeficiency Virus in Human Lymphocytes Is Overcome by the Viral Vif Protein

Madani, Navid, Kabat, David

The vif gene of human immunodeficiency virus type 1 (HIV-1) encodes a basic Mr 23,000 protein that is necessary for production of infectious virions by nonpermissive cells (human lymphocytes and...

A Comprehensive Approach to Mapping the Interacting Surfaces of Murine Amphotropic and Feline Subgroup B Leukemia Viruses with Their Cell Surface Receptors

Tailor, Chetankumar S., Nouri, Ali, Kabat, David

Because mutations in envelope glycoproteins of retroviruses or in their cell surface receptors can eliminate function by multiple mechanisms, it has been difficult to unambiguously identify sites for...

Cellular and Viral Specificities of Human Immunodeficiency Virus Type 1 Vif Protein

Madani, Navid, Kabat, David

The vif gene of human immunodeficiency virus type 1 (HIV-1) greatly enhances the infectivity of HIV-1 virions that are released from cells classified as nonpermissive (e.g., lymphocytes, macrophages,...

Cooperation of Multiple CCR5 Coreceptors Is Required for Infections by Human Immunodeficiency Virus Type 1

Kuhmann, Shawn E., Platt, Emily J., Kozak, Susan L., Kabat, David

In addition to the primary cell surface receptor CD4, CCR5 or another coreceptor is necessary for infections by human immunodeficiency virus type 1 (HIV-1), yet the mechanisms of coreceptor function...

Sodium-Dependent Neutral Amino Acid Transporter Type 1 Is an Auxiliary Receptor for Baboon Endogenous Retrovirus

Marin, Mariana, Tailor, Chetankumar S., Nouri, Ali, Kabat, David

The baboon endogenous retrovirus (BaEV) belongs to a large, widely dispersed interference group that includes the RD114 feline endogenous virus and primate type D retroviruses. Recently, we and...

Cellular and Species Resistance to Murine Amphotropic, Gibbon Ape, and Feline Subgroup C Leukemia Viruses Is Strongly Influenced by Receptor Expression Levels and by Receptor Masking Mechanisms

Tailor, Chetankumar S., Nouri, Ali, Kabat, David

Chinese hamster ovary (CHO) cells are resistant to infections by gibbon ape leukemia virus (GALV) and amphotropic murine leukemia virus (A-MLV) unless they are pretreated with tunicamycin, an...

A Putative Cell Surface Receptor for Anemia-Inducing Feline Leukemia Virus Subgroup C Is a Member of a Transporter Superfamily

Tailor, Chetankumar S., Willett, Brian J., Kabat, David

Domestic cats infected with the horizontally transmitted feline leukemia virus subgroup A (FeLV-A) often produce mutants (termed FeLV-C) that bind to a distinct cell surface receptor and cause severe...

Polymorphisms of the Cell Surface Receptor Control Mouse Susceptibilities to Xenotropic and Polytropic Leukemia Viruses

Marin, Mariana, Tailor, Chetankumar S., Nouri, Ali, Kozak, Susan L., Kabat, David

The differential susceptibilities of mouse strains to xenotropic and polytropic murine leukemia viruses (X-MLVs and P-MLVs, respectively) are poorly understood but may involve multiple mechanisms....

Frequent Substitution Polymorphisms in African Green Monkey CCR5 Cluster at Critical Sites for Infections by Simian Immunodeficiency Virus SIVagm, Implying Ancient Virus-Host Coevolution

Kuhmann, Shawn E., Madani, Navid, Diop, Ousmane M., Platt, Emily J., Morvan, Jacques, Müller-Trutwin, Michaela C., ...

In contrast to humans, several primate species are believed to have harbored simian immunodeficiency viruses (SIVs) since ancient times. In particular, the geographically dispersed species of African...

Adaptive Mutations in the V3 Loop of gp120 Enhance Fusogenicity of Human Immunodeficiency Virus Type 1 and Enable Use of a CCR5 Coreceptor That Lacks the Amino-Terminal Sulfated Region

Platt, Emily J., Kuhmann, Shawn E., Rose, Patrick P., Kabat, David

To identify sites in gp120 that interact with the CCR5 coreceptor and to analyze the mechanisms of infection, we selected variants of the CCR5-dependent JRCSF molecular clone of human...

Segregation of CD4 and CXCR4 into Distinct Lipid Microdomains in T Lymphocytes Suggests a Mechanism for Membrane Destabilization by Human Immunodeficiency Virus

Kozak, Susan L., Heard, Jean Michel, Kabat, David

Recent evidence has suggested that plasma membrane sphingolipids and cholesterol spontaneously coalesce into raft-like microdomains and that specific proteins, including CD4 and some other T-cell...

The Envelope Glycoprotein of Human Endogenous Retrovirus Type W Uses a Divergent Family of Amino Acid Transporters/Cell Surface Receptors

Lavillette, Dimitri, Marin, Mariana, Ruggieri, Alessia, Mallet, François, Cosset, François-Loïc, Kabat, David

The human endogenous retrovirus type W (HERV-W) family includes proviruses with intact protein-coding regions that appear to be under selection pressure, suggesting that some HERV-W proviruses may...

Implication of the Lymphocyte-Specific Nuclear Body Protein Sp140 in an Innate Response to Human Immunodeficiency Virus Type 1

Madani, Navid, Millette, Robert, Platt, Emily J., Marin, Mariana, Kozak, Susan L., Bloch, Donald B., ...

The viral infectivity factor (Vif) of human immunodeficiency virus type 1 (HIV-1) neutralizes an unidentified antiviral pathway that occurs only in nonpermissive (NP) cells. Using a yeast two-hybrid...

N-Linked Glycosylation and Sequence Changes in a Critical Negative Control Region of the ASCT1 and ASCT2 Neutral Amino Acid Transporters Determine Their Retroviral Receptor Functions

Marin, Mariana, Lavillette, Dimitri, Kelly, Sean M., Kabat, David

A widely dispersed interference group of retroviruses that includes the feline endogenous virus (RD114), baboon endogenous virus (BaEV), human endogenous virus type W (HERV-W), and type D primate...

Genetic Structure of Rauscher Spleen Focus-Forming Virus

Bestwick, Richard, Ruta, Martin, Kiessling, Ann, Faust, Charles, Linemeyer, David, Scolnick, Edward, ...

Rauscher spleen focus-forming virus contains functional gag and pol genes and a partially deleted env gene which is structurally related to the env genes of dual tropic murine leukemia viruses.

Unexpectedly Large Size of Globin Messenger Ribonucleic Acid

Gaskill, Pauline, Kabat, David

The globin messenger RNA of rabbit reticulocytes is identified on the basis of its presence in polysomes, absence from single ribosomes, and sedimentation coefficient of 9 S. Analysis of this RNA by...

Porcine Endogenous Retroviruses Infect Cells Lacking Cognate Receptors by an Alternative Pathway: Implications for Retrovirus Evolution and Xenotransplantation

Lavillette, Dimitri, Kabat, David

A PHQ motif near the amino termini of gammaretroviral envelope glycoprotein surface (SU) subunits is important for infectivity but not for incorporation into virions or binding to cognate receptors....

Synthesis and Glycosylation of Polyprotein Precursors to the Internal Core Proteins of Friend Murine Leukemia Virus

Evans, Leonard H., Dresler, Steven, Kabat, David

Synthesis and post-translational processing of murine leukemia virus proteins were analyzed in a murine cell line (Eveline) that produces large amounts of Friend lymphatic leukemia virus....

Kinetic Factors Control Efficiencies of Cell Entry, Efficacies of Entry Inhibitors, and Mechanisms of Adaptation of Human Immunodeficiency Virus†

Platt, Emily J., Durnin, James P., Kabat, David

Replication of human immunodeficiency virus type 1 (HIV-1) in diverse conditions limiting for viral entry into cells frequently leads to adaptive mutations in the V3 loop of the gp120 envelope...

Variants of Human Immunodeficiency Virus Type 1 That Efficiently Use CCR5 Lacking the Tyrosine-Sulfated Amino Terminus Have Adaptive Mutations in gp120, Including Loss of a Functional N-Glycan

Platt, Emily J., Shea, Danielle M., Rose, Patrick P., Kabat, David

By selecting the R5 human immunodeficiency virus type 1 (HIV-1) strain JR-CSF for efficient use of a CCR5 coreceptor with a badly damaged amino terminus [i.e., CCR5(Y14N)], we previously isolated...

Role of Low CD4 Levels in the Influence of Human Immunodeficiency Virus Type 1 Envelope V1 and V2 Regions on Entry and Spread in Macrophages

Walter, Brandon L., Wehrly, Kathy, Swanstrom, Ronald, Platt, Emily, Kabat, David, Chesebro, Bruce

Human immunodeficiency virus type 1 (HIV-1) isolates vary in their ability to infect macrophages. Previous experiments have mapped viral determinants of macrophage infectivity to the V3 hypervariable...

Cloning and characterization of a cell surface receptor for xenotropic and polytropic murine leukemia viruses

Tailor, Chetankumar S., Nouri, Ali, Lee, Chun G., Kozak, Christine, Kabat, David

Xenotropic and polytropic murine leukemia viruses (X-MLVs and P-MLVs) cross-interfere to various extents in non-mouse species and in wild Asian mice, suggesting that they might use a common receptor...

gp120 envelope glycoproteins of human immunodeficiency viruses competitively antagonize signaling by coreceptors CXCR4 and CCR5

Madani, Navid, Kozak, Susan L., Kavanaugh, Michael P., Kabat, David

Signal transductions by the dual-function CXCR4 and CCR5 chemokine receptors/HIV type 1 (HIV-1) coreceptors were electrophysiologically monitored in Xenopus laevis oocytes that also coexpressed the...

A Sodium-Dependent Neutral-Amino-Acid Transporter Mediates Infections of Feline and Baboon Endogenous Retroviruses and Simian Type D Retroviruses

Tailor, Chetankumar S., Nouri, Ali, Zhao, Yuan, Takeuchi, Yasuhiro, Kabat, David

The type D simian retroviruses cause immunosuppression in macaques and have been reported as a presumptive opportunistic infection in a patient with AIDS. Previous evidence based on viral...

Origin and Rapid Evolution of a Novel Murine Erythroleukemia Virus of the Spleen Focus-Forming Virus Family

Hoatlin, Maureen E., Gomez-Lucia, Esperanza, Lilly, Frank, Beckstead, Jay H., Kabat, David

The Friend spleen focus-forming virus (SFFV) env gene encodes a glycoprotein with apparent Mr of 55,000 that binds to erythropoietin receptors (EpoR) to stimulate erythroblastosis. A retroviral...

An Array of Novel Murine Spleen Focus-Forming Viruses That Activate the Erythropoietin Receptor

Gomez-Lucia, Esperanza, Zhi, Yu, Nabavi, Melud, Zhang, Weibin, Kabat, David, Hoatlin, Maureen E.

The Friend spleen focus-forming virus (SFFV) env gene encodes a 409-amino-acid glycoprotein with an apparent Mr of 55,000 (gp55) that binds to erythropoietin receptors (EpoR) to stimulate...

Effects of CCR5 and CD4 Cell Surface Concentrations on Infections by Macrophagetropic Isolates of Human Immunodeficiency Virus Type 1

Platt, Emily J., Wehrly, Kathy, Kuhmann, Shawn E., Chesebro, Bruce, Kabat, David

It has been proposed that changes in cell surface concentrations of coreceptors may control infections by human immunodeficiency virus type 1 (HIV-1), but the mechanisms of coreceptor function and...

An Endogenous Inhibitor of Human Immunodeficiency Virus in Human Lymphocytes Is Overcome by the Viral Vif Protein

Madani, Navid, Kabat, David

The vif gene of human immunodeficiency virus type 1 (HIV-1) encodes a basic Mr 23,000 protein that is necessary for production of infectious virions by nonpermissive cells (human lymphocytes and...

A Comprehensive Approach to Mapping the Interacting Surfaces of Murine Amphotropic and Feline Subgroup B Leukemia Viruses with Their Cell Surface Receptors

Tailor, Chetankumar S., Nouri, Ali, Kabat, David

Because mutations in envelope glycoproteins of retroviruses or in their cell surface receptors can eliminate function by multiple mechanisms, it has been difficult to unambiguously identify sites for...

Cellular and Viral Specificities of Human Immunodeficiency Virus Type 1 Vif Protein

Madani, Navid, Kabat, David

The vif gene of human immunodeficiency virus type 1 (HIV-1) greatly enhances the infectivity of HIV-1 virions that are released from cells classified as nonpermissive (e.g., lymphocytes, macrophages,...

Cooperation of Multiple CCR5 Coreceptors Is Required for Infections by Human Immunodeficiency Virus Type 1

Kuhmann, Shawn E., Platt, Emily J., Kozak, Susan L., Kabat, David

In addition to the primary cell surface receptor CD4, CCR5 or another coreceptor is necessary for infections by human immunodeficiency virus type 1 (HIV-1), yet the mechanisms of coreceptor function...

Sodium-Dependent Neutral Amino Acid Transporter Type 1 Is an Auxiliary Receptor for Baboon Endogenous Retrovirus

Marin, Mariana, Tailor, Chetankumar S., Nouri, Ali, Kabat, David

The baboon endogenous retrovirus (BaEV) belongs to a large, widely dispersed interference group that includes the RD114 feline endogenous virus and primate type D retroviruses. Recently, we and...

Cellular and Species Resistance to Murine Amphotropic, Gibbon Ape, and Feline Subgroup C Leukemia Viruses Is Strongly Influenced by Receptor Expression Levels and by Receptor Masking Mechanisms

Tailor, Chetankumar S., Nouri, Ali, Kabat, David

Chinese hamster ovary (CHO) cells are resistant to infections by gibbon ape leukemia virus (GALV) and amphotropic murine leukemia virus (A-MLV) unless they are pretreated with tunicamycin, an...

A Putative Cell Surface Receptor for Anemia-Inducing Feline Leukemia Virus Subgroup C Is a Member of a Transporter Superfamily

Tailor, Chetankumar S., Willett, Brian J., Kabat, David

Domestic cats infected with the horizontally transmitted feline leukemia virus subgroup A (FeLV-A) often produce mutants (termed FeLV-C) that bind to a distinct cell surface receptor and cause severe...

Polymorphisms of the Cell Surface Receptor Control Mouse Susceptibilities to Xenotropic and Polytropic Leukemia Viruses

Marin, Mariana, Tailor, Chetankumar S., Nouri, Ali, Kozak, Susan L., Kabat, David

The differential susceptibilities of mouse strains to xenotropic and polytropic murine leukemia viruses (X-MLVs and P-MLVs, respectively) are poorly understood but may involve multiple mechanisms....

Frequent Substitution Polymorphisms in African Green Monkey CCR5 Cluster at Critical Sites for Infections by Simian Immunodeficiency Virus SIVagm, Implying Ancient Virus-Host Coevolution

Kuhmann, Shawn E., Madani, Navid, Diop, Ousmane M., Platt, Emily J., Morvan, Jacques, Müller-Trutwin, Michaela C., ...

In contrast to humans, several primate species are believed to have harbored simian immunodeficiency viruses (SIVs) since ancient times. In particular, the geographically dispersed species of African...

Adaptive Mutations in the V3 Loop of gp120 Enhance Fusogenicity of Human Immunodeficiency Virus Type 1 and Enable Use of a CCR5 Coreceptor That Lacks the Amino-Terminal Sulfated Region

Platt, Emily J., Kuhmann, Shawn E., Rose, Patrick P., Kabat, David

To identify sites in gp120 that interact with the CCR5 coreceptor and to analyze the mechanisms of infection, we selected variants of the CCR5-dependent JRCSF molecular clone of human...

Segregation of CD4 and CXCR4 into Distinct Lipid Microdomains in T Lymphocytes Suggests a Mechanism for Membrane Destabilization by Human Immunodeficiency Virus

Kozak, Susan L., Heard, Jean Michel, Kabat, David

Recent evidence has suggested that plasma membrane sphingolipids and cholesterol spontaneously coalesce into raft-like microdomains and that specific proteins, including CD4 and some other T-cell...

The Envelope Glycoprotein of Human Endogenous Retrovirus Type W Uses a Divergent Family of Amino Acid Transporters/Cell Surface Receptors

Lavillette, Dimitri, Marin, Mariana, Ruggieri, Alessia, Mallet, François, Cosset, François-Loïc, Kabat, David

The human endogenous retrovirus type W (HERV-W) family includes proviruses with intact protein-coding regions that appear to be under selection pressure, suggesting that some HERV-W proviruses may...

Implication of the Lymphocyte-Specific Nuclear Body Protein Sp140 in an Innate Response to Human Immunodeficiency Virus Type 1

Madani, Navid, Millette, Robert, Platt, Emily J., Marin, Mariana, Kozak, Susan L., Bloch, Donald B., ...

The viral infectivity factor (Vif) of human immunodeficiency virus type 1 (HIV-1) neutralizes an unidentified antiviral pathway that occurs only in nonpermissive (NP) cells. Using a yeast two-hybrid...

N-Linked Glycosylation and Sequence Changes in a Critical Negative Control Region of the ASCT1 and ASCT2 Neutral Amino Acid Transporters Determine Their Retroviral Receptor Functions

Marin, Mariana, Lavillette, Dimitri, Kelly, Sean M., Kabat, David

A widely dispersed interference group of retroviruses that includes the feline endogenous virus (RD114), baboon endogenous virus (BaEV), human endogenous virus type W (HERV-W), and type D primate...

Genetic Structure of Rauscher Spleen Focus-Forming Virus

Bestwick, Richard, Ruta, Martin, Kiessling, Ann, Faust, Charles, Linemeyer, David, Scolnick, Edward, ...

Rauscher spleen focus-forming virus contains functional gag and pol genes and a partially deleted env gene which is structurally related to the env genes of dual tropic murine leukemia viruses.

Unexpectedly Large Size of Globin Messenger Ribonucleic Acid

Gaskill, Pauline, Kabat, David

The globin messenger RNA of rabbit reticulocytes is identified on the basis of its presence in polysomes, absence from single ribosomes, and sedimentation coefficient of 9 S. Analysis of this RNA by...

Porcine Endogenous Retroviruses Infect Cells Lacking Cognate Receptors by an Alternative Pathway: Implications for Retrovirus Evolution and Xenotransplantation

Lavillette, Dimitri, Kabat, David

A PHQ motif near the amino termini of gammaretroviral envelope glycoprotein surface (SU) subunits is important for infectivity but not for incorporation into virions or binding to cognate receptors....

Synthesis and Glycosylation of Polyprotein Precursors to the Internal Core Proteins of Friend Murine Leukemia Virus

Evans, Leonard H., Dresler, Steven, Kabat, David

Synthesis and post-translational processing of murine leukemia virus proteins were analyzed in a murine cell line (Eveline) that produces large amounts of Friend lymphatic leukemia virus....

Kinetic Factors Control Efficiencies of Cell Entry, Efficacies of Entry Inhibitors, and Mechanisms of Adaptation of Human Immunodeficiency Virus†

Platt, Emily J., Durnin, James P., Kabat, David

Replication of human immunodeficiency virus type 1 (HIV-1) in diverse conditions limiting for viral entry into cells frequently leads to adaptive mutations in the V3 loop of the gp120 envelope...

Variants of Human Immunodeficiency Virus Type 1 That Efficiently Use CCR5 Lacking the Tyrosine-Sulfated Amino Terminus Have Adaptive Mutations in gp120, Including Loss of a Functional N-Glycan

Platt, Emily J., Shea, Danielle M., Rose, Patrick P., Kabat, David

By selecting the R5 human immunodeficiency virus type 1 (HIV-1) strain JR-CSF for efficient use of a CCR5 coreceptor with a badly damaged amino terminus [i.e., CCR5(Y14N)], we previously isolated...

Role of Low CD4 Levels in the Influence of Human Immunodeficiency Virus Type 1 Envelope V1 and V2 Regions on Entry and Spread in Macrophages

Walter, Brandon L., Wehrly, Kathy, Swanstrom, Ronald, Platt, Emily, Kabat, David, Chesebro, Bruce

Human immunodeficiency virus type 1 (HIV-1) isolates vary in their ability to infect macrophages. Previous experiments have mapped viral determinants of macrophage infectivity to the V3 hypervariable...

Human Immunodeficiency Virus Type 1 Vif Functionally Interacts with Diverse APOBEC3 Cytidine Deaminases and Moves with Them between Cytoplasmic Sites of mRNA Metabolism▿

Marin, Mariana, Golem, Sheetal, Rose, Kristine M., Kozak, Susan L., Kabat, David

VifIIIB, which has been a standard model for the viral infectivity factor of human immunodeficiency virus type 1 (HIV-1), binds the cytidine deaminase APOBEC3G (A3G) and induces its degradation,...

Evidence that Ecotropic Murine Leukemia Virus Contamination in TZM-bl Cells Does Not Affect the Outcome of Neutralizing Antibody Assays with Human Immunodeficiency Virus Type 1 ▿

Platt, Emily J., Bilska, Miroslawa, Kozak, Susan L., Kabat, David, Montefiori, David C.

The TZM-bl cell line that is commonly used to assess neutralizing antibodies against human immunodeficiency virus type 1 (HIV-1) was recently reported to be contaminated with an ecotropic murine...