Calculating ratios of the ORs (OR for the association between exposure and p53 mutation–positive breast cancer divided by OR for the association between exposure and p53 mutation–negative breast cancer) as an indicator of heterogeneity of ORs across groups yielded statistically significant differences when examining active and passive smoking exposure [p53 mutation–positive cancer: OR = 1.55 (95% CI, 1.11–2.15); p53 mutation– negative cancer: OR = 0.77 (95% CI, 0.43–1.38); ratio of the ORs, 0.50 (95% CI, 0.27–0.93)]. This evidence concerns the gene TP53 and breast cancer.