Constructs of four different genetic loci were transfected into the avian leukosis virus-induced chicken B cell line DT40, which continues diversification of its rearranged light chain immunoglobulin gene by gene conversion. Analysis of stable transfectants revealed an unexpectedly high frequency of targeted integration into the homologous gene loci of DT40. Transcriptional activity of the target gene locus is not required, since a construct of the untranscribed ovalbumin gene also integrated predominantly by homologous recombination. A construct derived from the beta-actin locus was transfected into other chicken cell lines to determine the cell type specificity of the phenomenon. Targeted integration still occurred at high frequency in two other B cell lines that do not have the gene conversion activity. However, the ratios of targeted to random integration were reduced by at least one order of magnitude in three non-B cell lines.