A total of 770 breakpoints (80 of them identified by the authors) from unrelated patients with two-break rearrangements resulting in reciprocal translocations were studied to determine whether they were located preferentially. The distribution of breakpoints among the chromosome arms differs from that expected on the basis of their lengths, with more than expected on chromosome arms 4p, 9p, 9q, 13q, 18q, 21p, 21q, 22p, and 22q and fewer than expected on 1p, 1q, 3p, 3q, 5q, 6q, 7p, 12p, 16p, and the gonosomes. More breakpoints than expected occurred in the centromeric regions, and fewer in the median regions. Distribution of breakpoints within bands differed with the technique used: with G banding a many more breakpoints were localized in the light bands and fewer in the dark bands. With R banding no fewer than expected were present in the light bands and only slightly more were found in the dark bands.