DPP4 and plasma cell myeloma: Patients with other tumors have been studied; the serum DPP4 activity and overall percentages of CD26+ lymphocytes and CD26+ white blood cells have been measured in patients with hematological malignancies such as non-Hodgkin lymphoma (NHL), Hodgkin lymphoma (HL), leukemia, plasmacytoma, and multiple myeloma.