SF3B1 and myelodysplastic syndrome with single lineage dysplasia: At least 15% ring sideroblasts are required for the diagnosis of MDS with ring sideroblasts (MDS-RS) in cases lacking mutations in the spliceosome gene SF3B1. SF3B1 mutations are found in 60%-80% of patients with refractory anemia with ring sideroblasts (RARS) or RARS with thrombocytosis (RARS-T) and are associated with favorable prognosis [3].