JAM-4, also known as IGSF5, is a 93kD member of the CTX (Cortical Thymocyte Marker in Xenopus) subfamily of the Ig superfamily. It is a type I transmembrane protein with an N-terminal extracellular V-type and C2 type Ig domain separated by a J segment, a transmembrane domain and a cytoplasmic tail with a PDZ binding motif. JAM-4 is expressed in intestinal, renal, and mammary epithelial cells, spermatogonia, and hematopoietic stem cells. It is a cell adhesion molecule that mediates homophilic interaction. JAM-4 also interacts with the intracellular membrane-associated guanylate kinase inverted 1 (MAGI-1). An additional isoform containing a 100 residue insertion between Gln 125 and Asp 126 has been reported. Over the region used as immunogen, mouse JAM-4 shares 82% and 63% amino acid sequence identity with rat and human JAM-4, respectively.