pLXSN contains elements derived from Moloney murine leukemia virus (MoMuLV) and Moloney murine sarcoma virus (MoMuSV), and is designed for retroviral gene delivery and expression (1–3). Upon transfection into a packaging cell line, pLXSN can transiently express, or integrate and stably express, a transcript containing Y+ (the extended viral packaging signal) the gene of interest, and a selectable marker. The 5' viral LTR in this vector contains promoter/enhancer sequences that control expression of the gene of interest in the multiple cloning site. The SV40 early promoter (PSV40e) controls expression of the neomycin resistance gene (Neor), which allows antibiotic selection in eukaryotic cells. pLXSN also includes the Col E1 origin of replication and E. coli Ampr gene for propagation and antibiotic selection in bacteria.