CSMD1 CUB and Sushi multiple domains 1 is a protein that in humans is encoded by the CSMD1gene. It is a potential tumour suppressor, the deletion of which may result in head and neck carcinomas behaving more aggressively. 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.