Although disruption of histone modification patterns is a common hallmark of human cancer, our knowledge of the mechanistic role of histone-modifying enzymes in its generation is very limited. We have recently identified an inactivating mutation in the histone deacetylase-2 (HDAC2) in sporadic carcinomas with microsatellite instability and in tumors arising in individuals with hereditary nonpolyposis colorectal cancer syndrome. Since HDAC2 seems to be a central player in epigenetic gene repression, we wondered whether HDAC2-truncating mutations conferred a particular expression signature on these cancer cells. Using unsupervised clustering analysis in microsatellite-unstable colorectal cancer cell lines, we have found that HDAC2 mutant cells (RKO and Co115) show a characteristically different expression microarray signature from HDAC2 wild-type cells (HCT-116, SW48, HCT-15 and LoVo). HDAC2 mutant cells exhibit upregulation of tumor-promoting genes, such as those of tyrosine kinases, mediators of cell cycle progression and angiogenic factors. The overexpression of these genes is associated with a loss of HDAC2 recruitment and a gain of histone H4 hyperacetylation in their particular 5'-end promoters, as observed by chromatin immunoprecipitation. Transfection of wild-type HDAC2 in mutant cells reverted this epigenetic pattern by repressing the transforming genes in association with HDAC2 promoter occupancy. These results suggest a role for HDAC2 mutations in human tumorigenesis through the derepression of key genes from multiple cellular transformation pathways.