21Tyrosine kinase inhibitors (TKIs) targeting the BCR-ABL1 fusion protein, encoded by the 22 Philadelphia chromosome, have drastically improved the outcomes for patients with 23 chronic myeloid leukemia (CML). Although several real-time quantitative polymerase 24 chain reaction (RQ-PCR) kits for the detection of BCR-ABL1 transcripts are 25 commercially available, their accuracy and efficiency in laboratory practice require 26 reevaluation. We have developed a new in-house RQ-PCR method to detect minimal 27 residual disease (MRD) in CML cases. MRD was analyzed in 102 patients with CML 28 from the DOMEST study, a clinical trial to study the rationale for imatinib mesylate 29 discontinuation in Japan. The BCR-ABL1/ABL1 ratio was evaluated using the 30 international standard (IS) ratio, where IS < 0.01% was defined as a major molecular 31 response. At enrollment, BCR-ABL1 transcripts were undetectable in all samples using a 32 widely-applied RQ-PCR method performed in the commercial laboratory, BML (BML 33Inc., Tokyo, Japan); however, the in-house method detected the BCR-ABL1 transcripts in 34 five samples (5%) (mean IS ratio: 0.0062 ± 0.0010%). After discontinuation of imatinib,
35BCR-ABL1 transcripts were detected using the in-house RQ-PCR in 21 patients (21%) 36 that were not positive using the BML method. Nineteen samples were also tested using a 37 commercially available RQ-PCR assay kit with a detection limit of IS ratio, 0.0007% Page3 38 (ODK-1201, Otsuka Pharmaceutical Co., Tokyo, Japan). This method detected low levels 39 of BCR-ABL1 transcripts in 14 samples (74%), but scored negative for five samples (26%) 40 that were positive using the in-house method. These data suggest that our new in-house 41 RQ-PCR method is effective for monitoring MRD in CML. 42 43 44 48 (t(9;22)(q34;q11.2)), referred to as the Philadelphia chromosome, which generates BCR-49 ABL1 fusion transcripts. The BCR-ABL1 protein constitutively activates tyrosine kinase 50