To determine the reason behind the efficacy difference between the two-drug regimen of CDK 4/6 inhibitor combined with endocrine therapy and the three-drug regimen comprising CDK 4/6 inhibitor, endocrine therapy combined with neratinib in HR+/HER2-low breast cancer, we also analyzed HER2 mRNA levels. The gene discussed is ERBB2; the disease is breast carcinoma.