Plot of K_S vs. physical position on X chromosome for mouse and human X- and Y-linked gene pairs. Physical positions on the X chromosome are from the UCSC Genome Browser Web site (http://genome.ucsc.edu/), assembly February 2003. Mouse gametologs are marked by uppercase letters; human gametologs (Lahn and Page 1999) by lowercase letters. Human strata as identified by Lahn and Page (1999) are boxed and labeled. Mouse strata boundaries are marked by dotted lines. Human X chromosome genes cluster in strata by both their physical positions on the X and by their divergence from their gametologs (K_S). Mouse X chromosomal genes cluster in strata by degree of divergence from their Y-linked gametologs only, not by map position. A/a, Sox3/Sry; B/b, Rbmx/y; C/c, Smcx/y; D/d, Ube1x/Ube1y1; E/e, Usp9x/y; F/f, Zfx/y-2; G/g, Utx/y; H, Eif2s3x/y (mouse only).