CD38 and essential thrombocythemia: Furthermore, within MPN patients, allele burden is higher in granulocytes compared to CD34+ cells [38], only a minority of CD34+CD38− stem and progenitor cells bear the JAK2V617F mutation in many individuals with ET or PV [39]–[41], and CD34+ cells expressing the mutation failed to out-compete normal cells in transplantation experiments using immunodeficient mice [40],[42].