Autoimmune diseases such as type 1 diabetes and rheumatoid arthritis (RA) are caused when immune cells known as lymphocytes attack our own body tissues. Depletion of the B cell subset of lymphocytes using an antibody specific for the protein CD20 has been shown to be effective for the treatment of RA. The effectiveness of such an approach for the treatment of type I diabetes is not known, largely because the reagents to address this question in preclinical models have been lacking.