IGSF1 contained 12 Ig-like domains of the C2 (constant region type 2) type. In addition, IGSF1 has a signal sequence and a potential transmembrane domain. IGSF1 was expressed as a 4.7-kb mRNA in many of the tissues tested, with the highest expression in pancreas, testis, and fetal liver. Additional 2.8- and 5.5-kb transcripts were observed in heart and testis, respectively. The cDNAs encoded a predicted 1,327-amino acid protein that was designated 'immunoglobulin superfamily member-1' (IGSF1) because it contained 12 Ig-like domains of the C2 (constant region type 2) type. In addition, IGSF1 has a signal sequence and a potential transmembrane domain. Northern blot analysis revealed that IGSF1 was expressed as a 4.7-kb mRNA in many of the tissues tested, with the highest expression in pancreas, testis, and fetal liver.