This pharmacologic distinction is clinically relevant, as large trials of the JAK1/JAK2 inhibitor ruxolitinib have demonstrated robust hematocrit control, spleen volume reduction, and durable hematologic remission in patients with hydroxyurea‐resistant PV, confirming that effective management of PV requires meaningful JAK2 blockade [7]. The gene discussed is JAK1; the disease is acquired polycythemia vera.