This suggests further exploration of checkpoint inhibitors in TNBC, HER2+ and ER+ breast cancer subtypes, in both therapy-naive and therapy-refractory settings, should be undertaken to compare anti-PD-1 and anti-PD-L1 therapies for differential therapeutic efficacy, and assess if the biomarker described in this manuscript has capacity to identify checkpoint inhibitor responsive patients, alone or in combination with existing biomarkers. This evidence concerns the gene ERBB2 and breast cancer.