FIG. 4.

Kinetics of mammary tumor occurrence and histopathology of mammary tumors derived from MMTV/wild-type MT, MMTV/MT-Y250F, and MMTV/MT-Y315/22F transgenic animals. (A to D) Photomicrographs comparing the histology of invasive malignancies from a wild-type PyV MT (A), an MT-Y250F tumor (B), and an MT-Y315/322F tumor (D) (magnification, ×250) with that of a noninvasive dysplasia (C) (magnification, ×100). The preinvasive MT-Y315/322F dysplastic nodule (C) is found at the end of dialated ducts but is a more extreme form of the hyperplasia observed in Fig. 2E and represents the solid nest in Fig. 2F. (E) Age at which a mammary tumor is first palpable in each transgenic strain. Also shown are the number of animals analyzed for each strain (n) and the median age at which tumors are palpable (T₅₀).