The reciprocal Philadelphia translocation between chromosomes 9 and 22 [t(9;22)(q34;q11)] creates a BCR-ABL1 fusion protein that occurs in approximately 95% of cases of chronic myelogenous leukemia (CML), 15% of cases of adult acute lymphoblastic leukemia, and 5% of adult cases of acute myeloid leukemia. The BCR-ABL1 protein is a constitutively activated tyrosine kinase that induces and maintains the neoplastic phenotype in these leukemias. PCR-based methods to identify and quantitate the tumor-specific BCR-ABL1 RNA have been shown to be an ultrasensitive diagnostic/prognostic tool for Philadelphia-positive leukemias. A novel tyrosine kinase inhibitor (TKI), imatinib, has been confirmed as an effective targeted treatment in most CML patients. A consensus goal for TKI treatment is to achieve a major molecular response (MMR), defined as a 3-log (1,000-fold) reduction in BCR-ABL1 transcripts. Patients who achieve an MMR have been shown to have a significantly reduced risk of disease progression. Conversely, increasing post-therapy BCR-ABL1 RNA levels convey a significantly increased risk of disease progression. The early identification of these high-risk patients may allow early changes to the therapeutic strategy, before frank relapse. Thus, quantitative measurement of BCR-ABL1 transcripts in blood and bone marrow both aids in the initial diagnosis of CML and is essential for routine post-therapy minimal residual disease monitoring. We describe here a method for quantitating BCR-ABL1 transcripts in peripheral blood or bone marrow of CML patients using real-time quantitative reverse transcription PCR (RQ-PCR).