Intrachromosomal and interchromosomal segmental duplications on 21p. 21p sequence is shown in the center (21pjoin) with other chromosomal regions shown above and below. Intrachromosomal (blue) and interchromosomal (red) segmental duplications (>90% identity, >10 kb) between 21p and the human genome (hg18) are depicted. Purple bars indicate the position of centromeres on each chromosome.