CYLD was originally identified as a tumor suppressor that is mutated in familial cylindromatosis. Recent studies suggested a role for CYLD in nuclear factor-kappaB (NF-κB) regulation. NF-κB activation has been connected with multiple aspects of oncogenesis but the underlying molecular mechanisms of persistent NF-κB activation in tumors remain largely unknown. Thus, we evaluated CYLD transcription in different colon and hepatocellular carcinoma cell lines and tissue samples, respectively. CYLD was downregulated or lost in all tumor cell lines investigated as compared with primary human colonic epithelial cells and hepatocytes, respectively. Further, quantitative PCR analysis revealed reduced CYLD mRNA expression in most tumor samples compared with non-tumorous tissue. Analysis on protein level confirmed these findings. Functional assays with CYLD transfected cell lines revealed that CYLD expression decreased NF-κB activity. Thus, functional relevant loss of CYLD expression may contribute to tumor development and progression, and may provide a new target for therapeutic strategies.