XIAP and Wilson disease: A modifier gene is a gene that, when mutated, does not produce an effect on its own, but may produce or enhance clinical manifestations when coupled with another genetic mutation.13 Several genes have been proposed as potential modifier genes for WD, including ATOX1 (antioxidant copper chaperone 1)14, COMMD1 (copper metabolism domain-containing 1)15, XIAP (X-linked inhibitor of apoptosis)16, and HFE (hemochromatosis gene).17 However, none of these candidates has been confirmed by additional research.