Milk and Rat Mammary Tumors 65 Fig. 3. Immunostaining for ER. Nuclear staining indicates immuno- reactive ER. Nuclei counterstained with HE. A) Intraductal papil- loma, B) Fibroadenoma, C) Adenocarcinoma. ×100. Although the mechanisms by which milk could alter tumorigenesis are unknown, there are some hypotheses that have been put forth to suggest an increased risk of breast cancer associated with consumption of milk. Studies of Fig. 4. Immunostaining for PCNA. Nuclear staining indicates immunoreactive PCNA. Nuclei counterstained with haematoxylin. A) Intraductal papilloma. PCNA labeling index is low. B) Fibro- adenoma. PCNA labeling index is moderate. C) Adenocarcinoma. PCNA labeling index is high. ×100. dietary sources of nutrients estimate that the proportion of saturated fat in the diet that comes from dairy products is about 31% in the United States and 50% in Sweden [25, 30].