In a pooled analysis of tumor marker and epidemiological risk factor data from 35,568 invasive BC case patients from 34 studies participating in the BC Association Consortium, obesity (BMI ≥ 30 kg/m2) in women ≤50 years was found to be more frequent in ER−/PR− than in ER+/PR+ tumors (P = 1.10−7). This evidence concerns the gene ESR1 and obesity due to melanocortin 4 receptor deficiency.