Acquired sideroblastic anemia with unilineage dysplasia (WHO RARS) is a clonal stem cell disorder characterized by erythroid dysplasia, mitochondrial accumulation of mitochondrial ferritin, defective erythroid maturation and anemia. A fraction of these patients also show elevated platelet counts; since 2001 this has been defined as RARS with marked thrombocytosis (RARS-T). It has recently been described that around half of RARS-T patients, along with a small subset of other MDS and mixed myelodysplastic/ myeloproliferative disorders, carry the JAK2 mutation, and that MPL mutations are found in single patients. Clinically, RARS-T patients show features of both RARS, essential thrombocythmia (ET) and to some extent also myelofibrosis. However, the degree of anemia and overall survival is more similar to RARS than myeloproliferative disorders. The occurrence of JAK2 mutations and features of ET in RARS is too frequent to be the result of chance only, and it is possible that this link may provide a key to an increased understanding of the genetic abnormalities causing ring sideroblast formation.