The mouse major histocompatibility complex (MHC) contains many genes in addition to the classical immune response genes. We have screened overlapping cosmid clones covering 170 kb of the H-2K region for genes expressed in embryonal carcinoma (EC) cells. The Ke-3 gene (Abe et al. 1988) found in this region was further studied by Southern, Northern, and sequence analysis. It is an expressed, intron-containing locus encoding a mouse homolog of the bacterial ribosomal protein S13. This is the first non-organelle S13 homolog identified in metazoans, and its genomic location has been determined precisely.