The amino acid sequence of the rat 60S ribosomal subunit protein L10 was deduced from the sequence of nucleotides in two recombinant cDNAs and confirmed by determination of the NH2-terminal amino acid sequence in the protein. Ribosomal protein L10 has 213 amino acids (the NH2-terminal methionine is removed after translation of the mRNA); the molecular weight is 24,456. Hybridization of the cDNA to digests of nuclear DNA suggests that there are 8 to 10 copies of the L10 gene. The mRNA for the protein is about 900 nucleotides in length. Rat L10 is related to ribosomal proteins from other eukaryotes. Ribosomal protein L10 is, in addition, the mammalian homolog of the chicken Jun-binding protein and is nearly identical to a putative Wilms' tumor suppressor. This is a presumptive example, of which there are many others, of an extraribosomal function of a ribosomal protein.