Abstract We have shown that high-frequency phenotypic switching of a transfected gene is associated with alterations in chromatin structure. To examine this phenomenon further, a plasmid containing HSV thymidine kinase and human α- and γ-globin genes was transfected into mouse L cells. All three genes were expressed through utilization of their individual promoters. One of these cell lines was capable of switching to its TK − phenotype at high frequencies (8%–10%). The revertants (TK −) had no TK or globin transcripts, while the rerevertants (TK +) expressed all three genes at their original levels. We conclude that genes introduced into cells by ligated cotransfection can be regulated coordinately and that the unit of this regulated expression can be at least 20 kb long.