The MDM2 oncogene encodes a protein that is overexpressed and gene-amplified in several human tumors. In addition, more than 40 tumor MDM2 splice variants have been identified, although, their main biological function is still undefined. Project 1. MDM2-A is one of the most commonly occurring splice variants and therefore, an Mdm2-a transgenic mouse model was generated, and the in vivo function of MDM2-A was analyzed. Evaluation of transgenic embryos suggested that MDM2-A is lethal for mouse development. In vitro, MDM2-A mediated growth inhibition that depended upon p53 and p21Waf1/Cip1 expression. Nevertheless, a single wild-type Mdm2-a transgenic mouse line was generated. Mdm2-a transgenic mice did not show a tumorigenic phenotype; however, Mdm2- a transgenic mice showed reduced longevity probably due to the elevated p53 activity. Project 2. Because cellular localization of proteins may provide information regarding their potential function, the expression of splice variants MDM2-A, B and FB26, and their coexpression with p53, full-length MDM2 and ARF were evaluated. All three splice variants localized to the nucleus in addition to faint cytoplasmic expression of MDM2-A and B. Nuclear localization of MDM2-A and B was controlled by a previously uncharacterized nuclear localization signal 2 (NLS2). Nucleoplasmic localization of FB26 was mediated both by the well-characterized NLS1 and a potentially novel NLS3 that was generated by aberrant splicing. p53 and full-length MDM2 co-localized with the splice variants in the nucleus. ARF did not co-localize with the splice variants and was predominately expressed within the nucleoli. In conclusion, even though MDM2 splice variants are usually found in human tumors, not all of them display transforming activities. It is possible that some MDM2 splice variants contribute to the development of tumors while others may have been generated because of defective splicing machinery as a consequence of tumorigenesis.