A set of retroviral vectors is described whose capacity for high efficiency transduction of functional genes into undifferentiated murine embryonic and haematopoietic cells makes them ideally suited for preclinical studies with murine models. Multiple unique cloning sites permit insertion of genes into the vectors such that no selectable marker exists or either the neomycin phosphotransferase (neo) gene, the hygromycin B phosphotransferase (hph) gene or the puromycin N-acetyl transferase (pac) gene is included as a dominantly acting selectable marker. Because the sequences in the viral gag region shown to improve the encapsidation of viral RNA have been modified to prevent viral protein synthesis and all env sequences have been removed to eliminate helper virus production by homologous recombination with packaging DNA, these vectors might prove useful in human gene therapy protocols.