FLT3 and chronic myelogenous leukemia, BCR-ABL1 positive: Interestingly, although Smad3–Ser423/425 was also phosphorylated in ST-CML stem cells and in CD48+, multipotent progenitor (MPP; CD135+KLS+) and KLS− CML cells, Smad3–Ser208 phosphorylation was unique to the most primitive LT-CML stem cells, as was Smad3–Foxo3a interaction (Fig. 3e–g).