Circulating CD34⁺ cell counts in control subjects without JAK2 (V617F) mutant alleles and in patients with PV or post-PV MF carrying the mutation. Control subjects include 15 healthy individuals and 20 patients with secondary erythrocytosis who did not carry the JAK2 (V617F) mutation. The 39 patients with PV or post-PV MF are subdivided into 2 groups according to the percentage of JAK2 (V617F) mutant alleles in their granulocytes (≤ 50% versus > 50%). Cell counts are shown in a box plot where circles indicate outliers. The Kruskal-Wallis test showed significant differences between these groups (P < .001).