In cancers, loss of expression of genes occurs about 10 times more frequently by hypermethylation of promoter CpG islands than by mutations.  For instance, in colon tumors compared to adjacent normal-appearing colonic mucosa, about 600 to 800 heavily methylated CpG islands occur in promoters of genes in the tumors while these CpG islands are not methylated in the adjacent mucosa.  In contrast, as Vogelstein et al. point out, in a colorectal cancer there are typically only about 3 to 6 driver mutations and 33 to 66 hitchhiker or passenger mutations.