Retroviral gene transfer into diploid fibroblasts for gene therapy

A process of gene transfer into diploid fibroblasts in vitro, comprising the step of:

• genetically modifying diploid fibroblasts explanted from a mammalian subject by a process consisting essentially of introducing into said fibroblasts a retroviral construct comprising a first nucleotide sequence encoding a first expression product, a viral long terminal repeat and a promoter sequence upstream of said first nucleotide sequence, and a viral long terminal repeat and a polyadenylation sequence downstream of said first nucleotide sequence, wherein said retroviral construct lacks one or more of the gag, pol, and env sequences required for retroviral replication, by contacting said fibroblasts with said retroviral construct in a virus-containing medium having a viral titer of at least about 10.sup.5 cfu/ml on NIH 3T3 fibroblasts to produce a population of fibroblasts at least 1% of which express said first expression product.