ESR1 and Decreased total neutrophil count: This study describes a 67-year-old female patient with estrogen receptor (ER)-positive, human epidermal growth factor receptor-2-positive breast cancer, undergoing chemotherapy with an epirubicin/cyclophosphamide (EC) regimen (epirubicin, cyclophosphamide) and pegylated G-CSF for neutropenia prophylaxis.