cfDNAs (n=41) obtained prior to T-DM1 treatment from HER2-positive patients enrolled in the LiqBreasTrack and LiqERBcept studies were orthogonally tested by the HER2/EFTUD2 dPCR assay and a targeted NGS panel (Oncomine Pan-Cancer Cell-Free Assay, Thermofisher). Here, ERBB2 is linked to cancer.