SF3B1 mutations in lymphoid and myeloid progenitors in MDS-RS. (A) Representative FACS analysis of BM MNCs from healthy NBM and SF3B1-mutated MDS-RS patient 6, showing gating strategy for viable Lin⁻CD34⁺CD19⁺ pro–B cells. (B) Mean (SEM) frequencies of pro–B cells in SF3B1-mutated MDS-RS patients (n = 9) and healthy NBM (n = 4). (C) VAF of SF3B1 mutations in MDS-RS pro–B cells (MDS-RS PROB, n = 5) and mature PB B cells from MDS-RS (MDS-RS B cells, n = 3) compared with MDS-RS bulk BM MNCs (n = 5). Healthy NBM pro–B cells (NBM PROB, n = 4) were included as a negative control for pyrosequencing. PAT, patient. (D) Representative gating strategy for FACS purification of PB CD3⁺CD8⁺ T cells, CD19⁺ B cells, and CD33/66B/15⁺ myeloid cells from MDS-RS patient 4. (E) VAF of identified mutations in FACS-purified PB myeloid, B, and T cells from MDS-RS patient 4 at diagnosis and 16 months later using droplet digital PCR. Healthy normal DNA was negative, as indicated by the dotted line (<0.1%).