Protocadherins are a large family of cadherin-like cell adhesion proteins that are involved in the establishment and maintenance of neuronal connections in the brain. There are three protocadherin gene clusters, designated alpha, beta and gamma, all of which contain multiple tandemly arranged genes. PCDHA9 (protocadherin alpha 9), also known as KIAA0345, is a 950 amino acid single-pass type I membrane protein that contains six cadherin domains and is encoded by a gene which is located within the protocadherin alpha gene cluster on human chromosome 5. Existing as multiple alternatively spliced isoforms, PCDHA9 functions as a potential calcium-dependent cell adhesion protein that may be involved in the establishment and maintenance of neuronal connections within the brain.