Structure of the breakpoint region on human 17q21.3 (A), chimpanzee 19q (B), human 17p13 (C), and chimpanzee 19p (D). The sequences with homology to human 17p13 are shown as blue lines, whereas regions of homology to 17q21.3 are in red. SmaI and EcoRI restriction sites are indicated. PCR products p40/41 and p210/211 were used as labeled probes in Southern blot hybridizations to isolate breakpoint-spanning SmaI fragments. Their positions are indicated by horizontal bars and the respective sizes of these fragments are added. The flanking genes, which have been identified by physical mapping and sequencing of cloned fragments overlapping the breakpoint-spanning SmaI fragments, are also indicated. E = EcoRI; S = SmaI.