Testing for association with GATE scores identified 16 putative core genes for rheumatoid arthritis outside the HLA region, of which six—TP53BP1, PDCD1, TNFRSF14, LAIR1, LILRA4, and IDO1—were supported by Mendelian randomization analysis based on the marginal likelihood of the causal effect parameter. This evidence concerns the gene IDO1 and rheumatoid arthritis.