The results suggested positive correlations between C9orf72 repeat expansions and the risk of Alzheimer's disease (OR = 6.36, 95% CI = 3.13–12.92, and p < 0.00001), while intermediate repeat copies of C9orf72 gene were not associated with the risk of the disease. C9orf72 repeat expansions were positively correlated with the risk of familial and sporadic ALS (OR = 293.25, 95% CI = 148.17–580.38, and p < 0.00001; OR = 35.57, 95% CI = 19.61–64.51, and p < 0.00001). Here, C9orf72 is linked to Alzheimer disease.