A phase II study (ClinicalTrials.gov identifier: NCT03321981) enrolled patients with HER2-low BC who were estrogen receptor (ER)-positive and progressed to cyclin-dependent kinase 4 and 6 (CDK4/6) and endocrine therapy, with an effective 24 week CBR of 16.7% for MCLA128 combined with endocrine therapy [70]. This evidence concerns the gene ESR1 and breast cancer.