Unusual molecular characteristics of a repeat sequence island within a Giemsa-positive band on the mouse X chromosome.

The mouse genome contains 50 copies of a long complex repeat unit localized as a repeat sequence island to the A3 Giemsa-positive (dark) band on the mouse X chromosome. The repeat units are not tandemly arranged but are juxtaposed and inserted by unrelated sequences of high repetition. The repeat sequence island possesses two notable features that have been suggested as diagnostic features of mammalian Giemsa-positive bands. First, the repeat sequence island encompasses a 1-megabase region devoid of CpG islands; second, it features a high concentration of L1 long interspersed repeat sequences.