In this study, we screened 34 snap frozen ovarian tumour samples for MDMX splice variants. In addition to MDMX-S, we identified numerous new MDMX transcripts in 16 of the 34 tumour samples. Two representatively chosen isoforms were characterized in more detail. We found that expression of the splice variant MDMX-211 – containing the complete RING-finger domain – in MCF-7 cells (wild-type p53) resulted in the stabilization of p53 and subsequently in the p53-mediated expression of p21. After treatment of these cells with cis-Platinum, a significant decrease of the p21-level was observed, suggesting a switch from cell cycle regulation to apoptosis. In contrast, the splice variant MDMX-Var6, with a complete p53-binding domain, does not have any impact on p21 expression. After addition of cis-Platin we observed complete suppression of the p53 induced p21 expression despite a strong increase of the p53-levels.