SF3B1 and myelodysplastic syndrome: A heterozygous Sf3b1+/− knockout mouse model has been shown to develop ring sideroblasts, suggesting that haploinsufficiency of SF3B1 may lead to their formation.11 Recent similar studies have not made this observation, however.14, 15 Thus, it is yet to be determined whether SF3B1 mutations found in MDS are loss-of-function mutations or gain/change-of-function mutations.