The clinical course was complicated by intermittent hydroxyurea non-adherence with marked hematocrit fluctuations; dose optimisation from 1 g to 2 g daily achieved stable hematological control.<h4>Conclusion</h4>This case adds to the emerging molecular landscape of JAK2-negative MPNs by identifying KMT2C loss-of-function as a clonal marker in a patient with a JAK2-negative PV-like MPN phenotype. This evidence concerns the gene JAK2 and acquired polycythemia vera.