APOE and Alzheimer disease: Number of APOE2, APOE3, and APOE4 carriers in each clinical diagnostic stage (a), in the two MCI substages (b), and in the three AD substages (c). The genotype of each individual is given in S1a Table. The P-values obtained using the Fisher’s exact test were 0.8841 (a), 0.08148 (b), and 0.4379 (c).