The cellular tandemization of the herpes simplex virus (HSV) thymidine kinase (tk) gene was studied in tk- mouse fibroblasts after gene transfer by microinjection into the nucleus or by calcium phosphate-mediated transfection. Three different DNA substrates, designed to yield simple integration patterns, were used: a gel-purified 3.6-kb Bam HI fragment containing the HSV tk gene; the same fragment self-ligated; and the 3.6-kb fragment ligated to a Bam HI-cleaved subset of genomic mouse DNA. The genomic DNA of six independently isolated transformed cell lines was analyzed by Southern blotting and the structure of the tk-specific DNA was studied. The data suggest that modifications (mutations, deletions, recombination events, and recircularization, etc.) of the input DNA fragment occur early after its introduction into the cell. Subsequently these structures are multiplied in a directional manner, generating larger arrays of DNA with distinct and regularly repeated areas. These concatemers can eventually be integrated into the host genome.