Based on its analysis of a large cohort of individuals homozygous for the ε4 variant of apolipoprotein E (APOE4), a multinational team of researchers is arguing that homozygosity for APOE4 should be considered a genetic form of Alzheimer’s disease. However, not everyone agrees that the findings warrant reclassifying APOE from risk factor to causal gene. Currently, APOE4 is classified as the strongest risk factor for developing AD. Another variant, the APOE2 variant, is protective, while APOE3 is neutral.