Loss of Cdk4 abrogates Ras-induced hyperplastic and dysplastic changes in the epithelial tissue of mammary glands and blocks breast tumor formation. Formalin-fixed paraffin-embedded mammary gland sections of Cdk4(neo/neo) (A), Cdk4(+/+) (B), Cdk4(neo/neo):MMTV-v-Ha-ras (C), and Cdk4(+/+):MMTV-v-Ha-ras (D) mice were deparaffinized and stained with H&E stain (40x). Arrows in (D) indicate the loss of ductal architecture. (E) H-Ras transgenic mice on Cdk4(neo/neo) and Cdk4(+/+) backgrounds were examined for breast tumor formation for a period of 65 weeks, and the number of mice that developed breast tumors was plotted against age. For these studies, 9 Cdk4(neo/neo):MMTV-v-Ha-ras mice and 10 Cdk4(+/+):MMTV-v-Ha-ras mice were used.