Recent data have shown that achievement of deeper molecular responses, molecular response 4 ([MR4] BCR–ABL1IS ≤0.01%) and molecular response 4.5 ([MR4.5] BCR–ABL1IS ≤0.0032%), results in better treatment outcomes in patients with CML (Falchi et al. 2013; Etienne et al. 2014; Hehlmann et al. 2014). This evidence concerns the gene BCR and chronic myelogenous leukemia, BCR-ABL1 positive.