Increased sister chromatid exchange events in the human late replicating X

Human female blood cultures were labeled with BrdU for detecting sister chromatid exchanges (SCEs) by the Hoechst 33258 fluorescence technique. Late labeling with 3H-thymidine and autoradiography allowed the identification of the late replicating X. The mean number of SCEs in the cells was 13. The isopycnotic X showed an exchange frequency according to its relative length in the karyotype; in the late replicating X a doubled number of SCE events was observed.