Abstract Purpose: Skp2 is a subunit of the SCF ubiquitin protein ligase, which plays a vital role in the control of tumorigenesis via its regulation of G1-S transition. Deregulation of Skp2 in various types of cancers correlates with aggressive clinical behavior and poor prognosis. Recent studies suggest that cell cycle–dependent fluctuation of Skp2 is governed by APCCdh1, another important E3 ligase, thereby preventing premature entry into S phase. To assess the potential role of APCCdh1 in tumorigenesis through proteolysis of Skp2, we have dissected the APCCdh1-Skp2 cascade. Experimental Design: We manipulated the APCCdh1-Skp2 cascade and examined its cellular behavior using both breast cancer and normal breast epithelial cells. Furthermore, applying immunohistochemistry, we analyzed the clinicopathologic significance of these molecules in patients with breast cancer. Results: Analysis of tissue arrays indicated that the percentage of samples positive for Cdh1 in breast cancer was significantly lower compared with normal breast tissues (P = 0.004). Conversely, the percentage of samples scored as positive for Skp2 in cancer was significantly higher than in normal tissues (P < 0.001). Moreover, prognostic studies revealed that relatively high levels of Cdh1 are associated with survivability in patients with breast cancer. In addition, depletion of Cdh1 by small interfering RNA in normal breast cells resulted in increased cellular proliferation, whereas knockdown of Skp2 significantly suppressed growth in breast cancer cells. Conclusions: This study shows a correlation between Skp2 and APCCdh1 in breast cancer. Thus, Cdh1 may act as an important component in tumor suppression and could be considered as a novel biomarker in breast cancer.