Ringtail lemur by San Francisco Bay artist, Pat Sherwood. Killian et al. demonstrate in this issue genomic imprinting divergence in the primate lineage roughly 75 million years ago. The divergence in M6P/IGF2R imprinting between primates and sister mammals, such as rodents and artiodactyls, provides an explanation for animal differences in carcinogen susceptibility. Moreover, it predicts that the success of in vitro embryo procedures such as cloning will be species-dependent.