The retinoblastoma gene 1 (RB1) is a putative biomarker for BC treatment, as its genomic alteration appears to be associated with CDK4/6 inhibitor resistance [45,46]; in the PALOMA-3 trial, loss of RB1 detected by basally derived ct-DNA from 195 patients was associated with worse PFS in patients treated with palbociclib and fulvestrant [41]. The gene discussed is CDK4; the disease is breast cancer.