APOE ε4 allele is a risk factor for cognitive impairment, and this risk is lower in MCI (OR = 2.53, 95%CI 1.69-3.79 p < 0.001) than in AD (OR = 4.37, 95%CI 3.02-6.33 p < 0.001). The gene discussed is APOE; the disease is Cognitive impairment.