Method of activating expression of and amplifying an endogenous gene in genomic DNA of a vertebrate cell which is not expressed in the cell as obtained or is not expressed at significant levels in the cell as obtained, comprising the steps of: (a) transfecting cells with DNA sequences comprising: 1) exogenous DNA sequences which repair, alter, delete or replace a sequence present in the cell or which are regulatory sequences not normally functionally linked to the endogenous gene in the cell as obtained; 2) DNA sequences homologous with genomic DNA sequences at a preselected site in the cells; and 3) amplifiable DNA encoding a selectable marker; (b) maintaining the cells under conditions appropriate for homologous recombination to occur between DNA sequences homologous with genomic DNA sequences and genomic DNA sequences; (c) culturing the homologously recombinant cells produced in (b) under conditions which select from amplification of the amplifiable DNA encoding a selectable marker, whereby the amplifiable DNA encoding a selectable marker and the endogenous gene functionally linked to exogenous DNA of (a) 1) are coamplified.