The exact underlying cause of pilomatrixoma is not well understood. Changes (mutations) in the CTNNB1 gene are found in at least 75% of isolated (without other signs and symptoms) pilomatrixomas. These mutations are somatic, which means they are not inherited and are only present in the tumor cells. The CTNNB1 gene encodes a protein that is needed to regulate cell growth and attachment. When the gene is not working properly, it can result in abnormal cell growth.