Attempts to explain the left-sided predominance by examining risk factors for breast cancer, including ethnicity [8, 16], morphological type [9], race [14, 16], age [8, 9, 13–16, 24], height and weight [8], age at menarche [8, 13], family history [8], handedness [8, 13, 23–26], reproductive history [15], lactation history [19], marital status [8, 14], estrogen receptor status [8, 14, 15], parous status [8, 12, 13, 15], menopausal status [8, 13], and breast size [8, 12], have failed to consistently account for the higher incidence of left-sided breast cancer. This evidence concerns the gene ESR1 and breast carcinoma.