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. This evidence concerns the gene TPO and Decreased total neutrophil count.