The CUB and sushi domain-containing proteins, CSMD1-3, are membrane proteins that are involved in cell-cell adhesion and are weakly expressed in most tissues, with higher levels of expression observed in the cerebellum and hippocampus. CSMD1 is part of the complement system that defends against pathogens through either the classical pathway or the alternative pathway. Located primarily in nerve growth cones, CSMD1 blocks the classical pathway of the immune system and is thought to be involved in tumor suppression, as defects in the gene encoding CSMD1 are associated with squamous cell carcinomas. CSMD2 and CSMD3 are located primarily in the brain and are implicated in some forms of head and neck cancer. Additionally, the CSMD3 gene is a candidate for induction of epileptic seizures.