Tissues were collected from mice at day 10 of lactation,after schedule 1 cervical dislocation. The left lower mammary gland was dissected out and placed in 4% para-formaldehyde overnight at 4uC on a rocking platform. The para-formaldehyde was then removed and the tissues were washed, first, in PBS for 30 minutes at 4uC, then in 30% ethanol for 15 min. at room temperature. This was followed by two 30 min. washes in 70% ethanol at room temperature. The tissues were then transferred to fresh 70% ethanol until ready for processing for histology. The tissues were dehydrated through a series of alcohol dilutions followed by penetration by wax under vacuum (in a Shandon Hypercentre). The tissues were then embedded in wax blocks for sectioning. Each sample was sectioned at three intervals at least 100 mm apart and a 5 mm section mounted for immuno-histochemistry analysis at each interval.