CML is perhaps unique amongst cancers in that clinical response and disease burden can be accurately monitored by a relatively easily-measured molecular marker, the BCR-ABL transcript.14 Regular polymerase chain reaction (PCR) monitoring is mandated by current international guidelines39,40 as a means of allowing clinicians (and patients) to track progress against defined milestones of disease response. This evidence concerns the gene ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive.