Aberrant DNA methylation is associated with many types of human cancers. To identify genes silenced in human colorectal cancers, we performed a microarray analysis for genes whose expression was induced by treatment of HCT116 human colon cancer cells with a demethylating agent, 5-aza-2'-deoxycitidine (5-aza-dC). Seven known genes were identified as being upregulated (> or =8-fold) and expressed at more than twice as high as the average level. Among these was the UCHL1 gene (also known as PGP9.5), which is involved in regulation of cellular ubiquitin levels. A dense CpG island in its promoter region was completely methylated in HCT116 cells, and no mRNA was detected. 5-Aza-dC treatment of HCT116 cells induced dose-dependent demethylation of the CpG island, and restored UCHL1 mRNA and protein expression. UCHL1 silencing was observed in 11 of 12 human colorectal cancer cell lines, and its methylation was detected in 8 of 17 primary colorectal cancers. Further, UCHL1 silencing was observed in 6 of 13 ovarian cancer cell lines, and its methylation was detected in 1 of 17 primary ovarian cancers. These results showed that UCHL1 is inactivated in human colorectal and ovarian cancers by its promoter methylation, and suggest that disturbance of cellular ubiquitin levels is present.