The present invention relates to transfected primary and secondary somatic cells of vertebrate origin, particularly mammalian origin transfected with exogenous genetic material (DNA) which encodes a desired (e.g. a therapeutic) product or is itself a desired (e.g. therapeutic) product, methods by which primary and secondary cells are transfected to include exogenous genetic material, including DNA targeting by homologous recombination, methods of producing clonal cell strains or heterogenous cell strains, methods of gene therapy in which the transfected primary or secondary cells are used, and methods of producing antibodies using the transfected primary or secondary cells. The present invention includes primary and secondary somatic cells, such as fibroblasts, keratinocytes, epithelial cells, endothelial cells, glial cells, neural cells, formed elements of the blood, muscle cells, other somatic cells which can be cultured and somatic cell precursors, which have been transfected with exogenous DNA which is stably integrated into their genomes or is expressed in the cells episomally. The exogenous DNA either encodes a product, such as a translational product (e.g. a protein) or a transcriptional product (e.g. a ribozyme or an anti-sense nucleic acid sequence) which is a therapeutic product or is itself a therapeutic product (e.g. DNA which binds to a cellular regulatory protein or alters gene expression).