Multiparous mammary glands of wild type and NN60 transgenic mice. Mammary gland whole mounts taken from wild type (A and C) and transgenic (B and D) animals 17 days after weaning of their second litter. At the lower portion of 10× magnification (A and B), proliferation of lobuloalveoar structures can be seen throughout the gland. Higher magnification of 30× (C and D) reveals slightly dilated ducts and numerous clusters on each branching duct of the NN60 mouse. (E-G) Hematoxylin and eosin staining of sections of small tumor foci found in the third thoracic mammary gland as seen at a magnification of 200×.