(A–D) LacZ staining of whole mount (A,C) and sectioned (B,D) mammary glands isolated from heterozygous 6 and 8 week old virgin female carrying LacZ reporter gene in the Twsg1 locus. (A) Twsg1 is present in ducts and terminal end buds (TEBs) at 6 weeks (B) and in a subset of body cells in the canalizing TEB. (C) Twsg1 expression is present in the mature ducts (2 mm scale bar). (D) This expression is observed primarily in the myoepithelium at 8 weeks (E) RT-PCR showing temporal expression of BMP pathway members in MGs at different postnatal developmental time points. Actb, beta-actin, Chrd, chordin; Chrdl1, chordin-like 1; Chrdl2, chordin-like 2; Cv2, crossveinless 2.