In this regard, we note that the most important genetic risk factor for developing SAD is a variant of apolipoprotein E (ApoE), a protein required to ferry cholesterol within lipoprotein particles: the ε4 allele (ApoE4) confers a significantly higher risk of developing AD than does the ε3 allele (ApoE3)152. The gene discussed is APOE; the disease is Alzheimer disease.