Cancer epigenetics is the study of epigenetic modifications to the DNA of cancer cells that do not involve a change in the nucleotide sequence, but instead involve a change in the way the genetic code is expressed. Epigenetic alterations may be just as important, or even more important, than genetic mutations in a cell's transformation to cancer. In cancers, loss of expression of genes occurs about 10 times more frequently by transcription silencing (caused by epigenetic promoter hypermethylation of CpG islands) than by mutations.