Fig. 1. Idiogram of the X chromosome showing the approximate locations of linkage markers and several X-linked ocular diseases. RP, retinitis pigmentosa; DXS, F9, known X-chromosome linkage markers. (Modified from Mackey DA, Buttery RG, Wise GM et al: Description of X-linked megalocornea with identification of gene locus. Arch Ophthalmol 109:829, 1991.)