CD34 and acquired polycythemia vera: Most interestingly, CCR2 expression is a unique characteristic of PMF CD34+ cells, since the receptor is not expressed by other Philadelphia-negative MPN subtypes, namely PV and ET, which share with PMF the same driver mutations (% of CD34+/CCR2+ cells: 0.98 ± 0.27, p < 0.001 vs. PMF, Figure 3A).