Using two-sample MR, we confirm a significant causal association between serum CD33 (myeloid cell surface antigen CD33) and Alzheimer’s disease35 (AD; BETA = 0.0091; SE = 0.0017; inverse variance-weighted [IVW] Padj = 3.62 × 10−4) (Figs. 3 and 4a). Here, CD33 is linked to Alzheimer disease.