Circulating CD34⁺ cells in patients with MPNs. Flow cytometry enumeration of circulating CD34⁺ cells in patients with PV (n = 239), ET (n = 391), or PMF (n = 106). Data are shown in a box plot depicting the upper and lower adjacent values (highest and lowest horizontal line, respectively), upper and lower quartile with median value (box), and outside values (dots). Values found in PMF patients are significantly higher than those found in PV or ET patients (P < .0001): the existence of overlaps is consistent with the notion that abnormal stem cell trafficking may be found also in some patients with PV or ET, especially those with advance disease. Overall, the available evidence indicates that flow cytometry enumeration of circulating CD34⁺ cells represents a simple, useful tool for estimating abnormal stem cell trafficking in patients with MPNs.