Immunohistochemical analyses for Id-1 in mouse mammary glands during various phases of development. Mammary glands from (A) early pregnant, (B) lactating, (C) lactational involuting and (D) pubertal mice were analyzed for the presence of Id-1 as described in the text. Note that, in all the mammary ducts, immunoreactivity is present only in myoepithelial cells and not in the luminal cells. Immunoreactivity is also not detected in the terminal end bud of the pubertal mouse. Original magnification, 400 ×.