Somatic SF3B1 mutations are present in >90% of refractory anemia with ringed sideroblasts (RARS) MDS patients as well as ∼70% of patients with non-RARS forms of MDS with RSs, such as refractory cytopenia with multilineage dysplasias and ringed sideroblasts (RCMD-RS) and RARS associated with marked thrombocytosis (RARS-T). The gene discussed is SF3B1; the disease is myelodysplastic syndrome with single lineage dysplasia.