RB1 and neoplasm: For patient 1, CTCs consistently recapitulated the sCNAs detected in the primary tumour FFPE DNA (amplification of the region containing CDK6 and deletion of the region containing RB1), that was undetected in the majority of cfDNA samples suggesting that CTCs may provide a more faithful reflection of tumour biology; in this case intrinsic resistance to CDK4/6 inhibitors from the time of primary surgery [29].