SF3B1 and X-linked retinoschisis: While many mutations may coexist in MDS and other neoplasms, splice factor mutations occur in a mutually exclusive manner [4,6] Remarkably, mutations in SF3B1 occur in up to 85% of sideroblastic MDS (MDS-RS) [4,5,7], a subtype of MDS characterized by the presence of ring sideroblasts (RS) in the bone marrow, and associated with good prognosis.