Epo hypersensitivity of erythroid progenitors from JAK2 617V>F mutated ET and PV patients. CD34⁺CD38⁺ cells from JAK2 617V>F–positive and normal bone marrow were seeded in methylcellulose with various concentrations of Epo. Erythroid colonies were counted, individually picked, and genotyped by allele-specific real-time PCR. (A) Mean numbers of wild-type (□), heterozygous (▩), and homozygous (■) mutated erythroid colonies in cultures seeded with 1500 CD34⁺CD38⁺ cells from the bone marrow of 5 ET patients, 5 PV patients, and 3 normal subjects (N) in the presence of various concentrations of Epo. Error bars indicate SEM. (B) Dose-response curves for erythroid progenitors with respect to their JAK2 617V>F genotype. The relative percentage of colonies is the mean number of colonies of each genotype at a given Epo concentration relative to that at the maximum Epo concentration (0.5 IU/mL). ■, JAK2 617V>F homozygous colonies; ▩, heterozygous colonies; □, wild-type colonies from ET and PV patients; ○, normal colonies from healthy bone marrow. Error bars represent SEM.