BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Most laboratories performing molecular AML MRD testing use the ABL1 reference gene to normalize testing results; however, a small subset uses alternative reference genes, such as GUSB, B2M and HMBS. The use of non-ABL1 reference genes by around 15% of laboratories has proven problematic to the standardization of BCR::ABL1 in CML, with conversion factors relating to GUSB being shown to be potentially more unstable when compared with their ABL1 counterparts despite extensive standardization [14].