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Mantle Cell Lymphoma (MCL) represents 5–10% of all non-Hodgkins lymphomas, making it an uncommon but difficult form of lymphoma to treat. It has the worst prognosis among the B cell lymphomas with median survival of three years. The genetic hallmark of MCL is the t(11,14)(q13;32) translocation causing amplification of cyclin D1 (CCND1). It is a well known cell cycle regulator. Multiple reports have shown a truncation in the cyclin D1 mRNA 3′ untranslated region. This truncation increases CCND1 protein expression by not only enhancing the half-life of CCND1 mRNA, but also evades microRNA regulation of mRNA translation. The dramatic overexpression of CCND1 mRNA and protein has been associated to poor clinical outcome in patients. We hypothesize that this truncation leads to a more aggressive phenotype and induces chemoresistance in MCL. We have identified 4 MCL cell lines (Granta-519, JVM-2, Jeko-1, and Z138) with different levels of the truncated CCND1 mRNA. We were able to show that Z138 and Jeko-1 have a much higher ratio of truncated CCND1 mRNA to the full length CCND1 mRNA as compared to Granta-519 and JVM-2. We were also able to show that this truncated mRNA leads to an increase in CCND1 protein expression. By using flow cytometry, we correlated the increase in CCND1 protein expression to faster cell cycle progression. We proposed that cell lines with increased CCND1 expression are phenotypically more aggressive, and would be able to continue cell cycle progression without serum support. We were able to arrest JVM-2 in G1 phase after 48 hours of serum starvation. However, we were not able to arrest cell cycle progression in Jeko-1 even after 96 hours of serum starvation. Western blot analysis shows that CCND1 protein expression is decreased in JVM-2 but remains unchanged in Jeko-1 with serum starvation. The same phenomenon was observed in Granta-519 and Z138. The MCL cell lines (Jeko-1 and Z-138) with more CCND1 protein expression were able to continue cell cycle progression in serum free media. The MCL cell lines (JVM-2 and Granta-519) with less CCND1 protein expression were not able to continue cell cycle progression in serum free media. This shows that CCND1 overexpression is associated with a more aggressive phenotype. We then treated the 4 MCL cell lines with varying concentrations of doxorubicin, a standard anthracycline chemotherapy used in the treatment of MCL patients. We used MTS assay to assess cell proliferation after treatment with doxorubicin. We found the IC 50 (inhibitory concentration 50%) of doxorubicin in these cell lines varied from 6nM to 600nM. The cell lines (Jeko-1 and Z-138) with more CCND1 protein expression have a much higher IC 50 as compared to the cell lines (JVM-2 and Granta-519) with less CCND1 protein expression. This demonstrates that CCND1 overexpression is associated with chemoresistance. We conclude truncation in CCND1 mRNA leads to increased CCND1 protein expression and faster cell cycle progression CCND1 overexpression is associated with an aggressive phenotype CCND1 overexpression is associated with chemoresistance.