TPO and neutropenia: Since JAK2 is downstream of erythropoietin (EPO), granulocyte-macrophage colony-stimulating factor (GM-CSF), and thrombopoietin (TPO), neutropenia has been found in clinical trials with the JAK inhibitors baricitinib (25, 26) and tofacitinib (27, 28) both of which target JAK2 to some extent.