When the immune system misfires, it can cause very different rare diseases that, on the surface, don’t seem related at all. Sanofi, though, is exploring similarities in rare autoimmune conditions that may allow it to treat a number of different disorders with a single therapy. We spoke to Pablo Sardi, head of rare disease research at Sanofi, about the company’s oral BTK inhibitor rilzabrutinib, the challenges of testing one drug in different rare diseases, and how this kind of approach might push doctors to focus less on symptoms and more on the root causes of a rare disease.