BCR-ABL transcript-level monitoring (BCR – gene encoding the break point cluster region protein; ABL – Abelson murine leukemia viral oncogene homolg 1) is a highly useful diagnostic tool that controls the effectiveness of the CML treatment and indicates at an early stage resistance development or disease progression. The gene discussed is BCR; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.