MDM4 and neoplasm: The decreased MDA-MB-231 CXCR4 expression in MDMX knockdown tumors correlated with the reduction of tumor metastasis for MDA-MB-231 cells, but MDM2 knockdown reduced MDA-MB-231 metastasis without a correlated reduction in CXCR4. Furthermore, because we did not see a change in CXCR4 or PTGS2 in the cell lines following MDM2 or MDMX knockdown, we reasoned that these genes were not direct targets and were only targets in the context of the animal model (Fig. 4b).