Germline mutations in the cylindromatosis (CYLD) gene have been described in families with cylindromas, trichoepitheliomas, and/or spiradenomas. Brooke-Spiegler syndrome (BSS) is the autosomal dominant predisposition to skin appendageal neoplasms including cylindromas, trichoepitheliomas, and/or spiradenomas. We review the clinical features, molecular genetics, and the animal models of BSS. To date, a total of 51 germline CYLD mutations have been reported, occurring in exons 9-20, in 73 families with diverse ethnic and racial backgrounds. Of 51 mutations, 86% are expected to lead to truncated proteins. The seven missense mutations reported to date occur only within the ubiquitin (Ub)-specific protease (USP) domain of the CYLD protein and most are associated exclusively with multiple familial trichoepithelioma (MFT). CYLD functions as a tumor suppressor gene. CYLD encodes a deubiquitinating (DUB) enzyme that negatively regulates the nuclear factor (NF)-kappaB and c-Jun N-terminal kinase (JNK) pathways. CYLD DUB activity is highly specific for lysine 63 (K63)-linked Ub chains but has been shown to act on K48-linked Ub chains as well. In 2008, the CYLD USP domain was crystallized, revealing that the truncated Fingers subdomain confers CYLD's unique specificity for K63-linked Ub chains. Recent work using animal models revealed new roles for CYLD in immunity, lipid metabolism, spermatogenesis, osteoclastogenesis, antimicrobial defense, and inflammation.