Effect of EPO concentrations on the ability of the anti-JAK2 inhibitor AZ960 to inhibit erythroid colony formation from 3 PV patients. CD34⁺ cells from PV patients were plated (2 × 10³ cells) in semisolid cultures in the presence of SCF, IL-3, and different concentrations of EPO and AZ960, as indicated. Colonies were counted after 14 days of incubation. (A) Typical results obtained from PV3 patient cells showing inhibition of colony formation by AZ960. (B) Mean IC₅₀ (± SE) of AZ960 on colony formation with different concentrations of EPO, as indicated, obtained from PV1, PV2, and PV3 patient cells. IC₅₀ were determined using Prism4 software analysis.