The groundbreaking work that led to the identification of the BCR-ABL TK protein as the driver of CML (Daley, Van Etten, & Baltimore, 1990) made it possible to develop a polymerase chain reaction (PCR) assay that measures the amount of BCR-ABL messenger RNA (mRNA) in blood cells. This evidence concerns the gene BCR and chronic myelogenous leukemia, BCR-ABL1 positive.