Among the 4 BC subtypes, notably, in luminal B breast cancer, MTHFR CT and TT genotypes showed a strong association with diabetes mellitus (OR = 7.22; 95% CI: 1.56–33.64; P = 0.0092) and a higher BMI (OR = 2.51; 95% CI: 1.1–5.79; P = 0.0289). The gene discussed is MTHFR; the disease is diabetes mellitus.