Detection of mutant alleles using ddPCR. Detection of mutant alleles using ddPCR. Genomic DNA from a FFPE sample is compartmentalized into millions of droplets containing 0 or 1 copy of target DNA (T790M mutant or wild type). After PCR cycling, droplets containing the endpoint fluorescence from each probe targeting mutant or wild-type DNA are detected using a flow cytometer. The detection of these signals allows quantification of allelic prevalence. Assays were considered “positive” if the measured event was ≥10 events/assay. FFPE, formalin-fixed paraffin-embedded; WT, wild-type; FAM, 6-carboxyfluorescein; TET, tetrachlorofluorescein. Masaru Watanabe et al. Clin Cancer Res 2015;21:3552-3560 ©2015 by American Association for Cancer Research