Ig (immunoglobulin) superfamily members exhibit functional characteristics including immune responses, growth factor signaling and cell adhesion. IGSF9 (immunoglobulin superfamily, member 9), also known as Nrt1 or IGSF9A, is a 1,179 amino acid single-pass type I membrane protein expressed in a wide variety of fetal tissues at eight and fourteen weeks of gestation. Belonging to the immunoglobulin superfamily and the Turtle family, IGSF9 is thought to play a role in dendrite outgrowth and synapse maturation. IGSF9 contains two fibronectin type-III domains and five Ig-like (immunoglobulin-like) domains. IGSF9 interacts with MAGI-2 and Shank 1, both of which contain SH3 (Src-homology 3) domains. Protein-protein interactions are central events in cellular signal transduction and are often mediated by SH3 domains. IGSF9 is encoded by a gene located on human chromosome 1q23.2 and mouse chromosome 1 H3.