The hematological follow-up of CML patients on TKI treatment is classically based on blood cell counts (estimation of the hematologic response), cytogenetic analysis by the determination of the percentage of Ph1-positive metaphases (evaluation of the cytogenetic response) and quantification of BCR-ABL mRNA transcripts (assessment of the molecular response). The gene discussed is ABL1; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.