Most first-line drugs for breast cancer are known to cause severe adverse effects, such as taxane-induced peripheral neuropathy, anthracyclines- or trastuzumab-induced cardiotoxicity, drug-induced alopecia, everolimus-induced mucositis, CDK4/6 inhibitor-induced neutropenia and diarrhea, PARP inhibitor-induced neutropenia and anemia, and receptor tyrosine kinase inhibitor-induced diarrhea [36]. The gene discussed is CDK4; the disease is anemia (phenotype).