In polycythemia vera (PV), where the Janus Kinase V617F mutation JAK2(V617F) is hyper sensitizing erythroid precursors to EPO, the depletion of CD169+ macrophages was found to reduce splenomegaly and reticulocytosis resulting in normalization of the erythroid compartment in mice (16). The gene discussed is SIGLEC1; the disease is acquired polycythemia vera.