A major limitation of gene therapy is the inability for specific cell types to be targeted for infection by viral vectors. This study shows the first identification of a new retroviral receptor protein, SLC35F2, selected by screening a random library of retroviral Envelope proteins for productive infection.The use of retroviral Envelope libraries randomized in the receptor binding domain have provided a major advance in targeted retroviral entry in the absence of known-targeting ligands (1). Viral-interference indicated the Env constructs utilize receptors distinct from the parental virus. In this report, the host cell receptor for the A5 viral Env isolate (2) has been cloned and identified as the solute carrier protein family 35 member F2 (SLC35F2) protein, a 10-membrane spanning protein homologous to other transporters of unknown function. Expression of the feline and human SLC35F2 cDNA in non-permissive cells renders the cells susceptible to infection by A5 virus with titers of 105.The use of alternative receptor proteins expressed on the surface of the target cells serves as a major breakthrough for gene delivery, as the range of potential receptors is now wide open and not limited to those defined by known retroviral receptors. This methodology does not require any prior knowledge of the target cell or complex engineering of known ligands into the Env structure. These studies have applications for gene therapy in cells that are poorly molecularly characterized, including stem cells.1. Bupp, K. & Roth, M.J. Targeting a retroviral vector in the absence of a known cell-targeting ligand. Human Gene Therapy 14, 1557-1564 (2003).2. Bupp, K., Sarangi, A. & Roth, M.J. Probing Sequence Variation in the Receptor-Targeting Domain of Feline Leukemia Virus Envelope Proteins with Peptide Display Libraries. J. Virol. 79, 1463-1469 (2005). A major limitation of gene therapy is the inability for specific cell types to be targeted for infection by viral vectors. This study shows the first identification of a new retroviral receptor protein, SLC35F2, selected by screening a random library of retroviral Envelope proteins for productive infection. The use of retroviral Envelope libraries randomized in the receptor binding domain have provided a major advance in targeted retroviral entry in the absence of known-targeting ligands (1). Viral-interference indicated the Env constructs utilize receptors distinct from the parental virus. In this report, the host cell receptor for the A5 viral Env isolate (2) has been cloned and identified as the solute carrier protein family 35 member F2 (SLC35F2) protein, a 10-membrane spanning protein homologous to other transporters of unknown function. Expression of the feline and human SLC35F2 cDNA in non-permissive cells renders the cells susceptible to infection by A5 virus with titers of 105. The use of alternative receptor proteins expressed on the surface of the target cells serves as a major breakthrough for gene delivery, as the range of potential receptors is now wide open and not limited to those defined by known retroviral receptors. This methodology does not require any prior knowledge of the target cell or complex engineering of known ligands into the Env structure. These studies have applications for gene therapy in cells that are poorly molecularly characterized, including stem cells. 1. Bupp, K. & Roth, M.J. Targeting a retroviral vector in the absence of a known cell-targeting ligand. Human Gene Therapy 14, 1557-1564 (2003). 2. Bupp, K., Sarangi, A. & Roth, M.J. Probing Sequence Variation in the Receptor-Targeting Domain of Feline Leukemia Virus Envelope Proteins with Peptide Display Libraries. J. Virol. 79, 1463-1469 (2005).