However, given its high sequence similarity and position immediately adjacent to the octapeptide-repeat array, as well as the observation that the presence, position and sequence of the nonapeptide unit are highly conserved among mammalian species [18], it is worth considering the hypothesis that the Gly54Ser substitution subtly alters the steric interaction of PrPC with copper, and that this altered interaction promotes the neuropathogenic effects of elevated brain copper levels in WD. The gene discussed is PRNP; the disease is Wilson disease.