The subject invention pertains to a transgenic animal model for tumorigenesis having a genome comprising a ras transgene and which is heterozygous or homozygous for a null p27 gene, and methods of using such animal to screen compounds or evaluate treatments for oncogenic and antitumor activity. The subject invention further concerns a transgenic mouse comprising a ras transgene and which has a genome that is wild-type p27.sup.+/+, and wherein the mouse has an FVB/N and C57BL/6X 129 genetic background; and methods of using such transgenic mice to screen compounds or evaluate treatments for oncogenic or antitumor activity. Advantageously, the female animals of the subject invention are fertile and capable of nursing their young. The subject invention also pertains to in vitro systems including isolated cells or tissues of animal models for tumorigenesis, which can be used to screen compounds and treatments for oncogenic and antitumor activity.