The pathognomonic genetic alteration in chronic myeloid leukemia is the formation of the BCR-ABL1 fusion gene, which produces a constitutively active tyrosine kinase that drives leukemic transformation. Targeted tyrosine kinase inhibitor treatment with imatinib, nilotinib, dasatinib, bosutinib, and ponatinib is the cornerstone of modern therapy for this hematologic malignancy. Real-time quantitative RT-PCR (RT-qPCR, also RQ-PCR) of BCR-ABL1 RNA is a necessary laboratory technique for monitoring the efficacy of tyrosine kinase inhibitor therapy and quantitatively assessing minimal residual disease. The molecular response measured by BCR-ABL1 RT-qPCR assists in identifying suboptimal responses and can help inform the decision to switch to alternative therapies that may be more efficacious (or to pursue more stringent monitoring). Furthermore, the tyrosine kinase inhibitor-mediated molecular response provides valuable risk stratification and prognostic information on long-term outcomes. Despite these attributes, informed, universal, practical utilization of this well-established monitoring test will require heightened efforts by the molecular diagnostics laboratory community to adopt the standardized reporting units of the International Scale. Without widespread adoption of the International Scale, the consensus major molecular response and early molecular response treatment thresholds will not be definable, and optimal clinical outcomes for patients with chronic myeloid leukemia may not be achieved.