SF3B1 and anemia (phenotype): MDS patients with RARS suffer from a refractory anemia and show erythroid hyperplasia and ineffective erythropoiesis as a result of increased apoptosis in the bone marrow.50 RS are characterized by excessive iron accumulation in the mitochondria of erythroid progenitors.9 The close association between SF3B1 mutations and RS is consistent with a causal relationship, and makes this the first gene to be strongly associated with a specific feature of MDS.5 How SF3B1 mutations affect formation of RS is still unknown.