CD34 and myelofibrosis: As the majority of healthy donor CD34+ lin– HSPCs did not express megakaryocyte genes and because mature megakaryocytes normally lose expression of CD34 during differentiation (Tomer, 2004), we hypothesized that aberrant co-expression of stem and progenitor and megakaryocyte surface antigens may enable the selective identification of myelofibrosis clone-derived HSPCs.