BTK and chronic graft versus host disease: As a central mediator of B-cell receptor signaling and microenvironment interactions, BTK supports survival, proliferation, and trafficking in multiple mature B-cell malignancies (mantle cell lymphoma, marginal zone lymphoma, Waldenström macroglobulinemia, and other indolent/aggressive lymphomas) and in selected immune-mediated conditions such as chronic graft-versus-host disease.