The WHO-HAEM4R entity “MDS/MPN with ring sideroblasts and thrombocytosis” (MDS/MPN-RT-T) has been largely redefined based on the highly prevalent SF3B1 mutation in these cases, and is renamed “MDS/MPN with SF3B1 mutation and thrombocytosis” in both WHO-HAEM5 and ICC. The gene discussed is SF3B1; the disease is thrombocytosis disease.