The first approach (top-down) consisted of taking the total cases presented by GLOBOCAN 2020 of breast cancer in Peru (22,486) and disaggregating them progressively by multiplying it by the following probabilities: (i) suffering HER2-positive breast cancer (25–30%), (ii) suffering metastatic breast cancer (9%) reported by FISSAL, and (iii) being treated by EsSalud (18%, estimated by considering the total breast cancer patients attended by Minsa, EsSalud, and INEN). This evidence concerns the gene ERBB2 and breast cancer.