Mutations found in human tumors often include transversions of GC to TA that may result from the mis-pairing of 8-oxoG with adenine during DNA replication. The human MutY (hMYH) enzyme, an adenine-specific DNA glycosylase, initiates repair at this mismatch. It has recently been demonstrated that inherited variants of hMYH may predispose individuals to multiple colorectal adenomas and carcinoma [Nat. Genet. 30 (2002) 227]. In this study, we demonstrate that two of these cancer-associated hMYH mutants, Y165C and G382D, are devoid of glycosylase activity directed towards 8-oxoG:A mispairs. These findings implicate a total loss of hMYH function associated with colorectal cancers.