New revised definitions were introduced by the CML Working Group of the ELN that take into account the sensitivity of the molecular qRT-PCR 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. Here, ABL1 is linked to chronic myelogenous leukemia, BCR-ABL1 positive.