Clinical features, drug therapies prior to RTX and at most recent followup, and serology are shown for patients with low serum IgM and for those retaining normal levels of IgM in Supplementary Tables 1 and 2, respectively (available on the Arthritis Care & Research web site at http://onlinelibrary.wiley.com/doi/10.1002/acr.22993/abstract). The gene discussed is CD40LG; the disease is Arthritis.