The fact that the absolute number of circulating CD26+ cells did not mirror BCR-ABL1 transcript levels as assessed by real time PCR of PB leukocytes, suggested that, in CML, minimal residual disease (MRD) assessment seems not to provide a reliable estimate of the actual residual LSC burden. The gene discussed is BCR; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.