ESR1 and breast cancer: The aim of the present proof-of-principle study was to evaluate the performance of a novel multiplex crystal digital PCR assay (12plex ESR1-AKT 6-color Crystal Digital PCR® assay Stilla Technologies, France), for the simultaneous detection in a single reaction of ten ESR1 mutations and AKT1 E17K in plasma-cfDNA samples from patients with ER​+ ​breast cancer.