Allelic imbalance on chromosome arm 22q has been detected in 50-70% of ovarian cancers, suggesting the presence of a tumor-suppressor gene on this chromosome arm that is involved in ovarian carcinogenesis. Recently, we isolated a candidate tumor-suppressor gene, MYO18B, at 22q12.1, which is deleted, mutated and hypermethylated in approximately 50% of lung cancers. In our study, we analyzed genetic and epigenetic alterations of the MYO18B gene in ovarian cancers. Missense MYO18B mutations were detected in 1 of 4 (25%) ovarian cancer cell lines and in 1 of 17 (5.9%) primary ovarian cancers. MYO18B expression was reduced in all 4 ovarian cancer cell lines and in 12 of 17 (71%) of primary ovarian cancers. MYO18B expression was restored by treatment with 5-aza-2'-deoxycytidine and/or trichostatin A in 3 of 4 cell lines with reduced MYO18B expression, and hypermethylation of the promoter CpG island for MYO18B was observed in 2 of these 3 cell lines. Its hypermethylation was also observed in 2 of 15 (13%) primary ovarian cancers. Thus, it was indicated that MYO18B expression is reduced in a considerable fraction of ovarian cancers by several mechanisms, including hypermethylation, while the MYO18B gene is mutated in a small subset of ovarian cancers. The present results suggest that MYO18B alterations, including both epigenetic and genetic alterations, play an important role in ovarian carcinogenesis.