Relationship between granulocyte JAK2 (V617F)-mutant allele burden and hematologic parameters in patients with ET or PV. The mutant allele burden was directly correlated with Hb level (ρ = 0.53, P < .001) and hematocrit (ρ = 0.61, P < .001), and inversely correlated with PLT count (ρ = −0.18, P < .001) and serum Epo level (ρ = −0.23, P < .001). These correlations suggest that the mutant allele burden is a determinant of the phenotypic features of JAK2 (V617F)-mutated MPN.