We have isolated and characterized a novel cDNA coding for a highly hydrophobic protein (B5) from a fetal mouse mandibular condyle cDNA library. The full-length mouse B5 cDNA is 3095 nucleotides long and contains a potential open reading frame coding for a protein of 705 amino acids with a calculated molecular weight of 80.5 kDa. The B5 mRNA is differentially polyadenylated, with the most abundant transcript having a length of 2.7 kb. The human homolog of B5 was isolated from a cDNA testis library. The predicted amino acid sequence of the human B5 is 98.5% identical to that of mouse. The most striking feature of the B5 protein is the presence of numerous (10-14) potential transmembrane domains, characteristic of an integral membrane protein. Similarity searches in public databanks reveal that B5 is 58% similar to the T12A2.2 gene of Caenorhabditis elegans and 60% similar to the STT3 gene of Saccharomyces cerevisiae. Furthermore, the report of an EST sequence (Accession No. Z13858) related to the human B5, but identical to the STT3 gene, indicates that B5 belongs to a larger gene family coding for novel putative transmembrane proteins. This family exhibits a remarkable degree of conservation in different species. The gene for B5, designated Itm1 (Integral membrane protein 1), is located on mouse chromosome 9.