Of the 15,823 genes that were tested for positive selection and had detectable expression in at least one tissue, 1,509 showed a strong preference for one tissue and were designated as tissue specific (c t .0.25 for some tissue t and c t .0.25 for all t9 ? t; see Methods). Based on this designation, spleen-and testes-specific genes were strongly enriched for PSGs: 22 of 174 (12.6%) spleenspecific genes were PSGs, compared with only 2.2% of other genes (P = 8.7610 211 , one-sided FET); and 45 of 715 (6.3%) testesspecific genes were PSGs, compared with 2.1% of other genes (P = 8.2610 210 ). There were also significant, but weaker, enrichments for PSGs among liver-specific (P = 9.1610 23 ) and breastspecific (P = 1.0610 22 ) genes. Not surprisingly, the spleen-specific PSGs generally appear to be immune-related, and many of the testes-specific PSGs are involved in spermatogenesis or sperm adhesion (they include ADAM2 and SPAM1; Table 3 ). The liver and breast specific genes are more heterogeneous. In contrast, only 2 of 254 (0.7%) cerebellum-specific genes were PSGs, compared with 2.3% of other genes (P = 0.066, one-sided FET). Only a few tissue-specific genes were identified by the clade tests, so it was not possible to compare the relationships between tissue-specific expression and positive selection in primates versus rodents. However, there were significant enrichments for primate PSGs among spleen-specific genes, and for rodent PSGs among testesspecific genes.