LTBP1 and breast cancer: The major findings of our study are that 1) Within the mammary epithelium, Ltbp1L is transcribed exclusively by ductal luminal cells and distinguishes them from the alveolar luminal lineage; 2) Ltbp1 protein and elastic fibers exclusively encase the ductal system; 3) Ltbp1L and 1S are upregulated during involution, a developmental window linked to high risk for breast cancer promotion; and 4) Ltbp1L is induced in mammary mesenchyme and sustained in the smooth-muscle cells of the nipple sphincter.