The full-length mouse B5 cDNA done was isolated from a fetal mouse mandibular condyle cDNA library (Hong el al.. 1996). This cDNA codes fora protein with multiple transmembranc domains, characteristic for an integral membrane pro tein. and the gene was therefore designated hm l Homology was found with the TI2A2. 2 gene of Caenorhabditis clegans and the S7T3 gene of Siicc/iaroniyces ccrevlsiac. The S lip pro tein is important lor N-linkod glycosylation and for assembly and functioning of oligosaccharyl transferase (Zuflerey et al.. 1995; Yoshida et al., 1995). The high level of homology between the three proteins over the whole molecule is remark able and might suggest a similar function for the hm l protein, although at present no biochemical evidence is available. The human honiologue for the hm l gene was isolated from a cDNA testis library (Hong et al.. 1996) and is highly iden tical (98.5%) to the mouse sequence at the amino acid level We now assigned, by in situ hybridization, the cosmid ICRFel07H0181 D, containing the human ITM1gene, to chro mosome hand I lq23. 3. This region has known synteny with a region of mouse chromosome 9 precisely where the mouse Imil gene has been mapped (Hong el al.. 1996). To obtain a more refined mapping of the gene, a chromosome Il-spccific YAC