The mdmx gene was shown to possess high homology to the mdm-2 gene and to encode a protein that can bind p53 and block p53 transactivation. Because Mdm-2 protein blocks the growth-suppressive activity of the p53 tumor-suppressor protein through similar activities, we examined the expression patterns of mdmx to determine how MdmX expression correlates with p53 protein levels. In this study, the expression pattern and protein levels of mdmx were examined in a number of cell culture systems. Like mdm-2, mdmx gene expression was constitutive during serum deprivation/restimulation of murine fibroblasts and differentiation of either murine teratocarcinoma or preadipocyte cells. In contrast, whereas mdm-2 gene expression was induced after cisplatin damage to ovarian carcinoma cells, mdmx expression remained constitutive. Because p53 transactivation is critical following a genotoxic stress, we examined p53:MdmX complexes after in vitro DNA-PK phosphorylation, a posttranslational modification that blocks p53 association with Mdm-2. The DNA-PK phosphorylation of p53 was capable of inhibiting p53:MdmX association. Thus, whereas DNA damage does not regulate mdmx mRNA levels, posttranslational modifications induced during DNA damage may block p53:MdmX association in vivo. These results demonstrate that, in the cell lines examined, mdmx gene expression remains constitutive during cell proliferation and differentiation or following DNA damage. Taken together, the data suggest that cells retain a constant level of MdmX. Thus, in undamaged cells, there exists the potential for an MdmX:p53 reservoir.