Abstract The amino acid sequence of the rat 40 S ribosomal subunit protein S4 was deduced from the sequence of nucleotides in a recombinant cDNA and confirmed from the NH 2-terminal amino-acid sequence of the protein. Ribosomal protein S4 has 282 amino acids (the NH 2-terminal methionine is removed after translation of the mRNA) and has a molecular weight of 31 841. Hybridization of the cDNA to digests of nuclear DNA suggest that there are 7–11 copies of the S4 gene. The mRNA for the protein is about 1 kb in length. Rat S4 is homologous to Saccharomyces cerevisiae YS6. The protein contains a possible internal duplication of 11 residues.