Abstract: We have developed a transcriptome-wide approach to identify genes affected by promoter CpG island hypermethylation and transcriptional silencing in colorectal cancer (CRC). By screening cell lines and validating tumor specific hypermethylation in a panel of primary human CRC samples, we estimate that nearly 5% of all known genes may be promoter methylated in an individual tumor. When directly compared to gene mutations, we find a much larger number of genes hypermethylated in individual tumors, and much higher frequency of hypermethylation within individual genes harboring either genetic or epigenetic changes. Thus, to enumerate the full spectrum of alterations in the human cancer genome, and facilitate the most efficacious grouping of tumors to identify cancer biomarkers and tailor therapeutic approaches, both genetic and epigenetic screens should be undertaken. The genes we identified can be used inter alia diagnostically to detect cancer, pre-cancer, and likelihood of developing cancer.