STAT5B and acquired polycythemia vera: To clarify a possible role of CIP2A in the constitutive phosphorylation of serine residues of STAT5 in primary JAK2V617F mutated cells, we measured CIP2A mRNA levels in granulocytes of MPN patients (3 PV, 9 MF) and healthy controls (n = 15) (Figure 4D).