Proteins of the major histocompatibility complex (MHC) are involved in cell-cell recognition: they bind and present antigens in the immune system. But, at least in mice, they are also involved in odor-based recognition between individuals. Mice tend to mate with MHC-dissimilar mice (to maintain MHC diversity) and nest with MHC-similar mice. In the September 12 Proceedings of the National Academy of Sciences Yamazaki et al. report that mothers recognize and preferentially retrieve MHC-similar pups, and that pups placed in a maze head for bedding soaked in the urine from an MHC-similar adult mouse (Proc Natl Acad Sci USA 2000, 97:10500-10502). The latter effect is partially, but not completely reversed by foster parenting, suggesting that some part of the response may be learned prenatally or by self-referral.