If we were to strictly apply an arbitrary cost-effectiveness threshold equal to the NZ gross domestic product per capita (2011 purchasing power parity [PPP]–adjusted: US$30,300; €23,700; £21,200), our study suggests that trastuzumab (2011 PPP-adjusted US$45,400/€35,900/£21,900 for 1 y at formulary prices) may not be cost-effective for ER+ (which are 61% of all) node-positive HER2+ early breast cancer patients but cost-effective for ER−/PR− subtypes (37% of all cases) to age 69 y. Here, ESR1 is linked to breast cancer.