(200) 28Recent studies indicate that 40% of chronic myeloid leukaemia (CML) patients who achieve 29 sustained undetectable BCR-ABL1 transcripts on tyrosine kinase inhibitor (TKI) therapy 30 remain disease-free after drug discontinuation. In contrast, 60% experience return of 31 detectable disease and have to re-start treatment, thus highlighting the need for an improved 32 method of identifying patients with the lowest likelihood of relapse. Here we describe the 33 validation of a personalised DNA-based digital PCR approach for quantifying very low 34 levels of residual disease, which involves the rapid identification of t(9;22) fusion junctions 35 using targeted next generation sequencing coupled with the use of a digital PCR (dPCR) 36 platform. t(9;22) genomic breakpoints were successfully mapped in samples from 32 of 32 37 patients with early disease. Disease quantification by DNA-dPCR was performed using the 38 Fluidigm BioMark platform on 46 follow-up samples from 6 of the 32 patients, including 39 36 samples that were in deep molecular remission (MR). Digital PCR detected persistent 40 disease in 81% of MR samples, out-performing both RT-dPCR (25%) and DNA-based 41 qPCR (19%). We conclude that dPCR for BCR-ABL1 DNA is the most sensitive available 42 method of residual disease detection in CML and may prove useful in the management of 43 TKI withdrawal. 44 45 3