The ability of mammalian cells to take up exogenously added DNA and to express genes included on that DNA has been well documented. DNA-mediated gene transfer (DMGT) potentially is a useful technique for the elucidation of many of the factors that control gene expression, and for the purification and isolation of mammalian genes. Before many of the benefits can be realized, however, a more detailed understanding of the organization, intracellular location, and expression of transferred genes will be needed. Recent studies have begun to characterize the DMGT process. Selected genes become linked to other exogenously added DNA during or subsequent to transfer and persist in the nuclei of recipient cells as part of large molecules called transgenomes. Transgenomes initially are maintained unstably and are lost from the population with first order kinetics. After a variable number of generations in culture, subpopulations arise that maintain the transferred genes stably. In these "stable" cells the transgenome is associated with a recipient cell chromosome, although the particular chromosome differs in independent "stable" lines. Mixture of an excess of specific nonselectable genes with the selected gene prior to transfer results in the inclusion of the nonselected genes in the transgenomes present in most cells that survive selection. This finding demonstrates the feasibility of introducing virtually any purified gene into mammalian cells. Recently microinjection of DNA directly into the nuclei of cells has been demonstrated. This technique greatly increases the frequency of gene transfer and significantly expands the number of cell types that can be genetically transformed.