All proteins analyzed in this study are either direct or indirect targets of novel anti-cancer drugs; thus, the dysregulation of wildtype and/or mutated genes was used as predictive markers whose corresponding protein could indicate a putative treatment option, i.e., PD-L1/MMR for immune checkpoint inhibitors, AR/ER for anti-hormone receptor treatment, Nectin-4 for anti-Nectin antibody-drug conjugates, Trop-2 for anti-Trop-2 antibody-drug conjugates, and tyrosine kinases (TK), including EGFR and HER2 for TK inhibitors (TKI). This evidence concerns the gene ESR1 and cancer.