Therefore, this paper aims to estimate the cost-effectiveness of a 12-mo trastuzumab regimen, compared to standard chemotherapy alone, by the four ER/PR subtypes (ER+/PR+ has the best prognosis; ER−/PR− has the worst prognosis) and then by age of treatment initiation (age groups from 25–29 to 90–94 y) in early stage HER2+ breast cancer that has spread regionally (node positive). The gene discussed is ESR1; the disease is breast cancer.