ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: Indeed, among the methodological limitations of the RQ-PCR, there may be sampling errors, low sensitivity, or low/absent transcriptional levels of BCR-ABL1 in the CML leukemic stem cell (LSC) that, when hidden in the hypoxic niche, does not synthetize the tyrosine kinase protein while it retains the fusion gene at DNA and RNA levels (44).