It is logical to assume that dysregulation or mutations in key mammary gland development regulators such as TBX3, FGFs, WNT signaling components, LEF1, BMP4, PTHrP, NRG3, ERBB4, and GLI3 during critical stages of mammary gland development may imprint lasting effects on the mammary gland structure and function, potentially predisposing individuals to heightened susceptibility to breast cancer later in life. The gene discussed is LEF1; the disease is breast carcinoma.