Brca1 expression in the mouse mammary gland. The line represents relative brca1 expression levels and is regulated in periodic waves during mammary gland development. Levels are low in the quiescent periods before puberty, in the virgin adult gland and in the resting gland. Levels are also low during lactational terminal differentiation. Levels are high during the proliferative periods at puberty and during pregnancy. Levels are also high during the apoptotic period of involution after weaning.