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Abstract 1328 Cks1 is an activator of the SCFSkp2 ubiquitin ligase complex that targets the cell cycle inhibitor p27Kip1 for degradation. Loss of Cks1 results in p27Kip1 accumulation and decreased proliferation. Cks1 expression is elevated in various B cell malignancies including Burkitt lymphoma and multiple myeloma. We have previously shown that loss of Cks1 results in elevated p27Kip1 levels and delayed tumor development in a mouse model of Myc-induced B cell lymphoma. Surprisingly, loss of Skp2 in the same mouse model also resulted in elevated p27Kip1 levels but exhibited no impact on tumor onset. Analysis of Cks1−/− murine embryonic fibroblasts (MEFs) unveiled a significant growth defect and comparison with double knock out Cks1−/−;p27Kip1−/− MEFs revealed a p27Kip1 independent defect at G1/S transition. These effects were associated with suppressed Cdk1 protein and Cdk2 activity in Cks1−/− cells. We hypothesized that Cks1 recruits further cell cycle regulators or facilitators of proliferation, which may explain the dramatic growth defect and the prolonged latency of lymphomagenesis upon loss of Cks1. Using Strep-Flag tagged Cks1 and tandem affinity purification followed by mass spectrometry we identified Geminin as a new Cks1 binding partner. Geminin is an inhibitor of rereplication and loss of Geminin leads to recurrent origin firing in S phase. The physical interaction of Cks1 and Geminin was present in cells of human and murine origin and was associated with Cdk2 binding to Geminin. Detailed analysis proved Cks1 dispensibel for cell cycle controlled Geminin degradation or phosphorylation. Surprisingly when mining public databases and RNA microarray data we found Geminin overexpressend in aggressive human lymphoma subtypes associated with high Ki-67 expression, and in murine Myc-induced lymphoma. Real time PCR revealed increased expression of Geminin during lymphomagenesis in Eμ-Myc mice, which correlated with protein expression. In conclusion, we established the physical Geminin-Cks1 protein interaction, present in human and murine cells, which could act in control of rereplication and genomic stability. Geminin is overexpressed in human and murine lymphoma revealing a potential role in lymphomagenesis. Disclosures: No relevant conflicts of interest to declare.