Immunohistochemical localization of PTEN in the mammary gland of MK-PTEN and WT mice. Sections from mammary glands removed from virgin, 9-day-pregnant, 14-day-pregnant, and 2-day-lactating mice were analyzed by immunohistochemistry as described in Methods. PTEN-expressing lobuloalveoli and ductal epithelial cells are indicated by black arrows and white arrows, respectively.