Rae-1 cDNA is one of the retinoic acid (RA) inducible cDNA clones in mouse embryonal carcinoma F9 cells. Rae-1 mRNAs were detected in mouse early embryos, but not in various tissues of adult mice. RAE-1 protein apparently consists of 253 amino acids and is likely to be a glycoprotein consisting of a leader sequence, an extracellular domain, a serine, threonine, proline-rich domain, and a transmembrane domain. Interestingly, it has a weak, but significant homology with major histocompatibility complex (MHC) class I molecules and was immunocytochemically identified as a cell surface protein. By determining partial nucleotide sequences of 17 Rae-1 cDNAs isolated from the RA-induced F9 cells, at least three different kinds of Rae-1 cDNAs were identified and were named Rae-1 alpha, Rae-1 beta, and Rae-1 gamma cDNAs, respectively. As the overall nucleotide sequence homology among these three cDNAs was about 98%, they constitute a novel gene family which is likely to be involved in early mammalian embryogenesis.