Abstract: A method for transferring a gene into target cells by a retrovirus with the use of serum-free medium. This method comprises infecting target cells with a retrovirus in serum-free medium optionally containing low-density lipoprotein and/or cytokines in the presence of a functional substance such as fibronectin in an amount effective in elevating the gene transfer efficiency of the retrovirus into the target cells by co-localizing the retrovirus and the target cells.

Abstract: A viral vector for the integration and expression of at least one heterologous gene in fowl pest cells consists wholly or in part of the proviral genome of fowl pest erythroblastosis or of a related virus in which said heterologous gene(s) replace(s) the v-erbA gene and/or the v-erB gene. Said gene(s) is(are) controlled either by an LTR promoter of the same virus, in which case the heterologous gene(s); mimics(s) the gene(s) it(they) replace, or by a heterologous promoter, in which case the additional att sequence is situated upstream of said heterologous promoter.