Bruton’s tyrosine kinase (BTK) plays a crucial role in the survival and proliferation of leukemic cells in many B-cell malignancies, including CLL, small lymphocytic leukemia (SLL), diffuse large B-cell lymphoma, Waldenstrom’s macroglobulinemia, mantle cell lymphoma, marginal zone lymphoma, as well as chronic graft versus host disease [2–5]. Here, BTK is linked to chronic graft versus host disease.