BCR and chronic myelogenous leukemia, BCR-ABL1 positive: With current qRT-PCR techniques being able to reliably detect up to a 5-log reduction in BCR-ABL1 using ABL1 or GUSB as control genes, the CML Working Group of the ELN has recently proposed revised definitions of molecular response that take into account the sensitivity of the molecular test, that is, MR4 indicates ≥ 4-log reduction (BCR-ABL IS ≤ 0.01%), MR4.5 indicates ≥ 4.5-log reduction (BCR-ABL IS ≤ 0.0032%), and MR5 indicates ≥ 5-log reduction (BCR-ABL IS ≤ 0.001%), especially in negative qRT-PCR results.