Sialoprotein cDNA was cloned from a 16-day mouse embryo library, and sequenced (GenBank accession number L20232). The cDNA encoded a protein of 324 amino acids. RNA was extracted from various cells and tissues of mice, but only the calvaria contained sialoprotein mRNA. The major mRNA species was 2.0 kb in length. Using RFLP analysis of DNA from interspecific crossbred mice, the gene for sialoprotein (Ibsp) was mapped to chromosome 5, with the gene order (Afpg, Mgsa)-Ibsp-Pdeb (where Afp is the alpha fetoprotein gene, Mgsa is a melanoma growth-stimulating activity gene and Pdeb is the phosphodiesterase β subunit gene).