Relative positions of genes studied herein, on mouse and human X chromosomes. The mouse X chromosome has been rearranged relative to the human X (Murphy et al. 1999). Map positions of human and mouse genes used in this study are indicated. Eif2s3x is not found in humans. Rbmx has not been mapped in mouse. UTX in human is placed on the chromosome relative to other genes according to radiation hybrid mapping (Lahn and Page 1999). Strata positions are marked along the human X chromosome (Lahn and Page 1999). Some X chromosomal genes from human were not used in this study because there were not orthologous X chromosomal genes in mouse, or because the mouse orthologs did not have Y chromosomal gametologs. Human and mouse map positions from: Human-Mouse Homology Map, Mouse Genome Database, http://www.informatics.jax.org/, February 2003.