Granulocyte mutant allele burden in JAK2 (V617F)-mutated and in CALR-mutated myeloid neoplasms. Data are shown in a box plot depicting the upper and lower adjacent values (highest and lowest horizontal line, respectively), upper and lower quartile with median value (box), and outside values (dots). (A) This analysis includes 250 patients with ET and 212 patients with PV at presentation, and 18 patients with post-ET myelofibrosis and 55 with post -PV myelofibrosis. In these JAK2 (V617F)-mutated myeloid neoplasms, progression from the primary disease to secondary myelofibrosis appears to be related to the mutant allele burden. In particular, the proportion of patients with values >50% increases progressively, indicating an increasingly higher proportion of cells that are homozygous for the mutation as a result of copy neutral loss of heterozygosity of chromosome 9p. Most patients with post-ET or post-PV myelofibrosis have values for granulocyte JAK2 (V617F)-mutant allele burden greater than 75%, consistent with a dominant population of homozygous cells. (B) This analysis includes 38 patients with ET at presentation and 10 patients with post-ET myelofibrosis. Also within these CALR-mutated myeloid neoplasms, progression to secondary myelofibrosis appears to be associated with a significant increase in the mutant allele burden. However, only 1 patient with post-ET myelofibrosis had a value consistent with a dominant population of homozygous cells. This might suggest that the higher mutant allele burden in patients with post-ET myelofibrosis most often reflects the progressive expansion of a heterozygous clone that eventually achieves full dominance in the bone marrow.