Genes and duplicons in genomic disorders involving simple duplicons. (a) Xp22.32 deletions in X-linked icthyosis; (b) reciprocal 17p12 duplications in Charcot-Marie Tooth syndrome 1A (CMT1A) and deletions in hereditary neuropathy with liability to pressure palsies (HNPP), (c) Xq28 inversions in hemophilia A; (d) 17q11.2 deletions in neurofibromatosis type 1 (NF1). Duplicons are shown as either triangles (when the orientation is known) or squares (when the orientation is unknown). Unequal crossing-over occurs primarily between duplicon copies depicted by shaded triangles. Percentages reflect the frequency of breakpoints at two alternative sites. Major genes responsible for each syndrome are shown as hatched rectangles. (cen) Centromere; (tel) telomere. (See text for references.)