RT Journal Article SR Electronic T1 mdmx Is a Negative Regulator of p53 Activity in Vivo JF Cancer Research JO Cancer Res FD American Association for Cancer Research SP 3221 OP 3225 VO 62 IS 11 A1 Finch, Rick A. A1 Donoviel, Dorit B. A1 Potter, David A1 Shi, Min A1 Fan, Amy A1 Freed, Deon D. A1 Wang, Ching-yun A1 Zambrowicz, Brian P. A1 Ramirez-Solis, Ramiro A1 Sands, Arthur T. A1 Zhang, Nan YR 2002 UL http://cancerres.aacrjournals.org/content/62/11/3221.abstract AB Regulation of p53 protein activity is required for normal embryogenesis, tumor suppression, and cellular response to DNA damage. Here we report that loss of mdmx, a p53-binding protein, results in midgestational embryo lethality, a phenotype that is completely rescued by the absence of p53. Mice homozygous for both mdmx and p53 null mutations are viable and appear developmentally normal. Fibroblasts derived from embryos with reduced mdmx expression demonstrate a decreased growth rate and increased UV-induced apoptosis compared with wild-type cells and contain elevated levels of p53 and several p53 target proteins including the proapoptotic bax protein. These observations demonstrate that mdmx functions as a critical negative regulator of p53 in vivo.