Ubiquitination of cyclin D1 signals for its proteosomal degradation. To assess the possibility that reduced cyclin D1 proteolysis is a putative mechanism for its accumulation during UVB-induced skin tumorigenesis, ubiquitination activity of cyclin D1 was assessed in UVB-induced murine SCCs. Cyclin D1 was rapidly ubiquitinated by control skin extract, whereas ubiquitination of cyclin D1 was significantly reduced in SCCs. Mutant cyclin D1, in which residues important for GSK3β-mediated degradation of cyclin D1 are altered to non-phosphorylatable alanine, was not ubiquitinated. We also observed phosphorylation-dependent inactivation of GSK3β in SCCs. Our results indicate reduced ubiquitination of cyclin D1 in UVB-induced murine SCCs and suggest that inactivation of GSK3β-dependent cyclin D1 degradation pathway contributes to the accumulation of cyclin D1 in UVB-induced murine SCCs. © 2002 Elsevier Science (USA). All rights reserved.