PRF1 and leukemia: Patients with fever, splenomegaly, thrombocytopenia and high levels of ferritin, in whom bone marrow aspirate does not document leukemia, should be rapidly screened by flow-cytometry for the expression of intracytoplasmic perforin and the ability to degranulate– as measured by surface expression of CD107a upon appropriate stimulation – of the peripheral blood lymphocytes [8].