JAK2 V617F-mutant patient HSCs proliferate faster than TET2 single-mutant and nonmutant cells. (A) Schematic of experimental design. Single Lin⁻CD34⁺CD38⁻CD90⁺CD45RA⁻ cells were sorted from MPN patient peripheral blood samples (n = 12; 3 essential thrombocythemia, 5 polycythemia vera, 4 secondary myelofibrosis) into 96-well plates, and cell counts were performed daily for 10 days. On days 9 to 10, clones were harvested and plated in a colony-forming cell assay, and the colonies produced were harvested for genotyping. Several patient assays were originally performed for a previous study⁴⁶  and were pooled together with newly generated data. (B-C) Human HSCs with a TET mutation alone (n = 2 patients, green line) have similar divisional kinetics to nonmutant HSCs (blue line), whereas cells with a JAK2 V617F mutation alone (n = 10 patients, red line) or both mutations (orange lines) had a reduced time to first (B) and second (C) divisions (higher proportion completed division at 72 hours; JAK, P < .01; JAK TET, P < .001). WT, n = 114; JAK, n = 108; TET, n = 40; JAK TET, n = 183. Patients with genotyped colonies: WT, n = 11; TET alone n = 2; JAK alone, n = 11; JAK TET together, n = 8. (D) Compared with WT cells, cells with TET mutations gave rise to a higher proportion of very small clones (<50 cells; P < .0001). WT, n = 114; JAK, n = 108; TET, n = 40; JAK TET, n = 183.