Base excision repair (BER) protects against damage to DNA from reactive oxygen species, methylation, deamination, hydroxylation and other by-products of cellular metabolism. Until last year, inherited deficiencies in the BER pathway had not been causally linked with any human genetic disorder. An apparent explanation was functional redundancy between proteins in this and other pathways. However, it was recently discovered that biallelic mutations in the BER DNA glycosylase MYH lead to an autosomal recessive syndrome of adenomatous colorectal polyposis and very high colorectal cancer risk. We review the molecular mechanism of tumourigenesis in MYH polyposis, the preliminary delineation of the MYH polyposis phenotype and the functional overlap of MYH with other repair proteins.