Expression of the cyclin-dependent kinase (Cdk) inhibitor (p27 super(Kip1)) is frequently reduced in human tumors, often correlating with poor prognosis. p27 super(Kip1) functions as a haploinsufficient tumor suppressor; however, the mechanism by which one allele of p27 super(Kip1) regulates oncogenic signaling in vivo is not well understood. We therefore investigated the mechanisms by which p27 super(Kip1) inhibits mammary tumor onset. Using the common background strain of FVB, p27 super(Kip1) heterozygosity (p27 super(+/-)) accelerated ErbB2-induced mammary tumorigenesis. We conducted microarray analyses of mammary tumors developing in mice with genetic haploinsufficiency for p27 super(Kip1) expressing a mammary-targeted ErbB2 oncogene. Global gene expression profiling and Western blot analysis of ErbB2/p27 super(+/-) tumors showed that the loss of p27 super(Kip1) induced genes promoting lymphangiogenesis, cellular proliferation, and collaborative oncogenic signaling (Wnt/ beta -catenin/Tcf, Cdc25a, Smad7, and Skp2). Skp2 expression was induced by ErbB2 and repressed by p27 super(Kip1). Degradation of p27 super(Kip1) involves an SCF-type E3 ubiquitin ligase, including Skp2. The Skp2 component of the SCF super(SKP2) complex that degrades p27 super(Kip1) was increased in ErbB2 tumors correlating with earlier tumor onset. In both murine and human ErbB2-overexpressing breast cancers, p27 super(Kip1) levels correlated inversely with Skp2. p27 super(Kip1) haploinsufficiency activated Wnt/ beta -catenin/hedgehog signaling. Reintroduction of p27 super(Kip1) inhibited beta -catenin induction of Tcf-responsive genes (Siamosis, c-Myc, and Smad7). p27 super(Kip1) is haploinsufficient for ErbB2 mammary tumor suppression in vivo and functions to repress collaborative oncogenic signals including Skp2 and Wnt/ beta -catenin signaling. (Cancer Res 2006; 66(17): 8529-41)