The CSF1R inhibitor JNJ-40346527 (3, Fig 1) completed a Phase II trial in disease modifying anti-rheumatic drug (DMARD)-refractory active RA but demonstrated a lack of efficacy in spite of sufficient exposure and systemic target engagement.[13] An anti-CSF1R antibody, FPA-008, is advancing to Phase II clinical trials for RA, and results of this study will help refute or support the role of CSF1R in RA disease. Here, CSF1R is linked to rheumatoid arthritis.