MegP is a pathophysiologically important population. (A) We analyzed MegP fractions in BM of 9 healthy donors and 17 patients with ET. Representative FACS profiles of control (bottom) and ET (top) samples are shown. MegPs were significantly expanded in ET BM. (B) Proportions of each HSPC fraction in CD34⁺ BM cells are shown. Bars indicate means of 9 healthy donors or 17 ET patients ± standard deviations (SDs). *P < .05 (Student t test). (C) The allele frequency of JAK2 V617F mutations in each HSPC population was evaluated. JAK2 V617F allele burden was significantly increased in the MegP fraction. Bars indicate means of 5 patient cases of ET ± SDs. *P < .05 (Tukey’s HSD test).