The FKBP-12-rapamycin associated protein (FRAP, also known as mTOR and RAFT-1) is a member of the phosphoinositide kinase related kinase family. FRAP has serine͞threonine kinase activity and mediates the cellular response to mitogens through signaling to p70s6 kinase (p70 s6k ) and 4E-BP1, resulting in an increase in translation of subsets of cellular mRNAs. Translational up-regulation is blocked by inactivation of FRAP signaling by rapamycin, resulting in G 1 cell cycle arrest. Rapamycin is used as an immunosuppressant for kidney transplants and is currently under investigation as an antiproliferative agent in tumors because of its ability to block FRAP activity. Although the role of FRAP has been extensively studied in vitro, characterization of mammalian FRAP function in vivo has been limited to the immune system and tumor models. Here we report the identification of a loss-of-function mutation in the mouse FRAP gene, which illustrates a requirement for FRAP activity in embryonic development. Our studies also determined that rapamycin treatment of the early embryo results in a phenotype indistinguishable from the FRAP mutant, demonstrating that rapamycin has teratogenic activity.