JAK2 and myeloproliferative neoplasm: This MPN driver mutation-negative LT-HSC was identified when a JAK2 V617F mutation-negative leukemic transformation was observed to occur in JAK2 V617F mutation-positive patients [2], and that LT-HSC could also be involved when phenotypical PV or ET develops in the absence of an MPN driver mutation (a “triple-negative MPN”).