Reports of single base-pair mutations within gene coding regions causing human genetic disease were collated. Thirty-five per cent of mutations were found to have occurred within CpG dinucleotides. Over 90% of these mutations were C----T or G----A transitions, which thus occur within coding regions at a frequency 42-fold higher than that predicted from random mutations. These findings are consistent with methylation-induced deamination of 5-methyl cytosine and suggest that methylation of DNA within coding regions may contribute significantly to the incidence of human genetic disease.