Increased frequencies of sister chromatid exchanges at common fragile sites (1)(q42) and (19)(q13).

Human lymphocyte cultures were treated with 5-azadeoxycytidine for the induction of the common fragile sites at 1q42 and 19q13 and with 5-bromodeoxyuridine for differential sister chromatid staining. A remarkably high frequency of sister chromatid exchanges was observed directly at the gaps of both fragile sites. In addition, the rate of sister chromatid exchanges occurring at the region corresponding to 1q42 without a concurrent visible gap was also increased. This confirms previous data on increased intrachromosomal recombination in common and rare fragile sites of various categories.