SF3B1 and myelodysplastic syndrome: Greater than 90% of refractory anemia with ringed sideroblasts (RARS) MDS patients and approximately 70% of non-RARS forms of MDS with ringed sideroblasts (i.e., refractory cytopenia with multilineage dysplasias; ringed sideroblasts and RARS associated with marked thrombocytosis) have mutations in SF3B1 (Inoue et al. 2016).