Flow cytometry-based assessment of intracellular WAS protein (WASP) is useful for screening patients suspected to have WAS or X-linked thrombocytopenia and neutropenia (233) and for following up chimerism after hematopoietic stem cell transplantation or somatic reversion mosaicism (234). The gene discussed is WAS; the disease is hereditary thrombocytopenia with normal platelets.