CCND1 and breast cancer: Neratinib can, through HER2’s 3’-UTR region binding hsa-miR-23a-5p and reducing HER2 mRNA stability, effectively downregulate Cyclin D1 and CDK4 by targeting the HER2 pathway, resulting in G1 arrest, and cancer cell proliferation, with enhanced efficacy of CDK4/6 inhibitors combined with endocrine therapy in HR+/HER2-low breast cancer.