The ApoEε4 alleleis further linked to an increased probability of developing AD.108−112 Why the ApoE gene is a risk factor for both heavy metal toxicityand AD is currently unclear, and various explanations have been proposed.108,113,114 The previously suggested hypothesisthat ApoE might bind and transport metal ions via Cys residues,105,106 which are present in ApoE2 and ApoE3 but not in ApoE4, has recentlybeen called into question.115. Here, APOE is linked to Alzheimer disease.