Progression of hormone-responsive cancers is characterized by deregulation of the cell cycle and cytoskeleton signaling. In addition, development of breast and endometrial cancer is influenced by the stimulatory action of estrogen. Up-regulation of dynein light chain 1 (DLC1), a component of cytoskeleton signaling, was recently found to promote tumorigenesis. The purpose of our study was to determine the role that DLC1 up-regulation plays in cell cycle progression. To achieve this goal, we used human breast ductal carcinoma ZR-75 cells overexpressing DLC1 as a model system. We found that ZR-75 cells with up-regulated DLC1 were hypersensitive to estrogen-dependent growth stimulation and that DLC1 had an accelerating effect on the G(1)-S transition and stimulated cyclin-dependent kinase 2 (Cdk2) activity. To better understand the promotion of the G(1)-S transition by DLC1, we sought to identify new DLC1-interacting proteins with roles in cell cycle regulation. Using a modified proteomic strategy, we identified two such DLC1-interacting proteins: Cdk2 and Cip-interacting zinc finger protein 1 (Ciz1). DLC1 was verified to interact with Cdk2 and Ciz1 in vivo. We also showed that down-regulation of DLC1 and Ciz1 reduced both Cdk2 activity and cell cycle progression of breast cancer ZR-75 and MCF-7 and endometrial Ishikawa cancer cells. Further, we showed that overexpression of DLC1 is accompanied by a reduction of nuclear p21(WAF1). These findings suggest that interactions among DLC1, Cdk2, and Ciz1 play a regulatory role in cell cycle progression of cancer cells presumably by influencing the levels of nuclear p21(WAF1).