Previous MR studies on Alzheimer’s disease10,12,25–33 suggest potential causal associations with BIN1, CCL27, C3, CD33, CD4 T cells, GDF-15 and SVEP1, whereas MR studies on Parkinson’s disease suggest a potential causal association with GPNMB, IL-6 and MIP1b. The gene discussed is C3; the disease is Parkinson disease.