Mutations in SF3B1 are frequent in patients with RARS and RARS-T and lead to an alteration in the BM ultrastructure. Left panel: We performed Sanger sequencing on exons 13, 14, 15, and 16 of SF3B1 in 456 patients, finding that the frequency of SF3B1 mutations is higher in patients with RARS (68%) and RARS-T (81%) compared with the other groups. White bars and black bars represent WT and mutant RARS and RARS-T patients, respectively. Right panel: Transmission electron microscopy of BM cells from a representative WT and mutant RARS patient. Arrows indicate the presence of abundant perinuclear iron deposits in the mutant compared with the WT patient. Specimens' cuts are 2-μm-thick sections.