Sex hormones, such as estrogen and testosterone, regulate sex-specific behaviors, but very few of the downstream genes carrying out the effects of sex hormones are known. Now, Nirao Shah and colleagues report an unbiased screen for genes with sexually dimorphic expression in the mouse hypothalamus and amygdala, validating 16 genes as having such an expression pattern through the use of microarrays and ISH (Cell 148, 596–607, 2012). The authors performed ISH in castrated male mice, where they observed changes in gene expression consistent with a regulatory role for testicular hormones, and reintroduction of testosterone into castrated mice restored the normal expression patterns of most genes. Mice with knockout alleles of four dimorphic genes were analyzed for a range of sex-typical behaviors in males and females. Males lacking Sytl4 sniffed females less but intromitted (penetrated) in more assays, while females lacking Irs4 or Cckar showed deficits in female-specific behaviors. Irs4-null mothers took longer to retrieve pups that wandered from the nest and also showed deficits in maternal aggression, attacking intruders in the cage less often than control mothers did. Cckar-null females allowed fewer intromissions from approaching males and were less receptive than control mice. These results show that aspects of sex-specific behavior are regulated by separable genetic pathways.