Fig. 2.

Sequence-based map of the pericentromeric region of human Xp. (A) The most proximal ≈650 kb of human Xp is depicted. The light gray bar at the left represents ≈150 kb of arm sequences, including the nearest expressed gene (ZXDA). The following blocks of satellite sequence are shown on the right: monomeric alpha satellite (red), HSAT4 (dark gray), and gamma satellite (yellow). The start of the X chromosome-specific higher-order alpha satellite (DXZ1) is shown in blue, with the light blue area reflecting monomers that lack higher-order structure but fall into the phylogenetic clade with DXZ1 monomers (see Fig. 3). The black diamond indicates the position of a single 880-bp block of nonsatellite sequence. Dotted lines indicate junctions between blocks of different satellite families (i.e., HSAT4 and alpha satellite) or junctions between blocks of the same family lying in opposite orientations. The arrowheads indicate the directionality of each satellite block. (B) Repetitive sequences are shown by using custom tracks from the UCSC Genome Browser (higher-order alpha satellite, blue; monomeric alpha satellite, red; HSAT4, dark gray; gamma satellite, yellow; LINEs, dark green; short interspersed nucleotide elements, light green). (C) The physical spans of the five satellite domains classified by phylogenetic analyses of alpha-satellite monomers (see Fig. 3) are indicated. Each L1 (LINE) repeat observed within proximal Xp and the alpha-satellite blocks is indicated as an oval, classified according to its subfamily [the currently active, human-specific elements (L1Hs) are shown at the top, with progressively older subfamilies listed below]. L1P subfamilies are primate-specific, whereas the L1M subfamily is quite large and composed of elements found throughout the mammalian radiation.