There is strong evidence that DNA methylation is not a neutral bystander in carcinogenesis, but actively contributes to the process. Methylation of cytosine is known to promote mutation to thymine, and there are many examples of tumours in which tumour suppressor proteins have been rendered functionless by methylation induced mutations of this kind. There is also evidence that tumour suppressor genes can be silenced epigenetically by de novo methylation of their CpG islands in the absence of any predisposing mutation. Although the experimental results in favour of the idea are becoming highly suggestive, it is too early to consider involvement of purely epigenetic processes as proven. New data bearing on this subject will doubtless be forthcoming.