The surface marker CD26 (dipeptidylpeptidase-IV) has been proposed as a specific marker of CML Leukemic Stem Cell (LSC) (155) and the assessment of CD26+ LSC in peripheral blood by flow cytometry has been suggested as a rapid tool for CML diagnosis (156) or as a marker for minimal residual disease during TKI treatment and treatment-free remission (157). This evidence concerns the gene DPP4 and chronic myelogenous leukemia, BCR-ABL1 positive.