Besides the class I genes that are highly polymorphic and encoded within the major histocompatibility complex (MHC), the genome of mammals contains class I genes that are oligo- or monomorphic. The latter category of class I genes, known as class Ib genes, encodes the neonatal intestinal Fc receptor (FcRn) involved in the transport of IgG from mother to offspring ; CD1 molecules involved in the presentation of glycolipids to T cells ; and zinc-alpha2-glycoprotein with no known function. The aims of the present study were to gain insights into the structure, function, and evolution of class Ib genes. The specific results obtained are as follows : 1) We cloned the genomic clones coding for the heavy chains of the human and mouse FcRn molecules, and determined the chromosomal localization of the genes. 2) We showed that the genetic region where the CD1 genes have been mapped is paralogous to the MHC.3) We showed that the mouse gene for zinc-alpha2-glycoprotein maps outside the MHC.4) We found that human CD1b molecules probably have no bound peptides.