Age at overweight onset (pheterogeneity 0.04), overweight duration (pheterogeneity 0.04) and the “Normal-overweight” (pheterogeneity 0.01) and “Normal-obesity” (pheterogeneity 0.01) trajectory were differentially associated with ER/PR-positive and ER/PR-negative breast cancer among never MHT users. This evidence concerns the gene ESR1 and obesity due to melanocortin 4 receptor deficiency.