The quantitative PCR assay generates variable BCR-ABL1 values. Quality control RNA samples with high and low BCR-ABL1 are included in every batch of patient samples in our laboratory to determine the run acceptability. The graphs demonstrate the BCR-ABL1 values recovered for the high (A) and low (B) controls analyzed over several months by different operators. These controls were manufactured in-house from BCR-ABL1–positive and –negative cell lines, and cells were stored frozen in RNA stabilization solution in large batches. Each control undergoes reverse transcription followed by quantitative polymerase chain reaction (PCR) in every batch of patient samples. Each value on the graphs represents the BCR-ABL1 result recovered from separate reverse transcription and quantitative PCRs. Reverse transcription is the process that introduces the greatest degree of variability. To appropriately evaluate the variability of an assay and to mimic the variability observed for patient BCR-ABL1 values, it is essential to include this process in the evaluation.