Although alterations in FGFR signaling pathway detected by tumor tissue analysis were shown to correlate with CDK4/6i resistance [298] and FGFR1 copy number alterations were detected in cfDNA of BC patients [326], the predictive value of FGFR copy number alterations in cfDNA for CDK4/6i has not been proven until now. This evidence concerns the gene CDK4 and breast cancer.