Expanded genomic organization of conserved mammalian MHC class I-related genes, human MR1 and its murine ortholog.

MR1 is a major histocompatibility complex (MHC) class I-related gene located outside the human MHC. Among several divergent class I molecules, the predicted MR1 molecule is closest, in the alpha1 and alpha2 domains, to the class I group to which the vertebrate classical class I molecules belong. We report here the genomic organizations of the human MR1 and mouse Mr1 genes. Both genes exhibit genomic structures largely similar to those of the MHC class I genes. However, they are highly expanded in their scale in contrast to the classical MHC class I genes. Inclusion of transposable elements into introns seems to partly contribute to these genomic structures. Several other MHC class I-related genes also show relatively large genomic structures. The present study extended heterogeneity in the genomic organization among the class I gene family by revealing a highly expanded structure of the human MR1 gene and its murine ortholog.