The TEC family kinase, BTK (Bruton’s tyrosine kinase) is best known as the target of ibrutinib (IMBRUVICA), the first-in-class covalent kinase active site inhibitor used to treat chronic lymphocytic leukemia, mantle cell lymphoma, Waldenström’s macroglobulinemia, and chronic graft-versus-host disease. Here, BTK is linked to chronic graft versus host disease.