TP53 and meningioma: While MDM2 and MDM4 mRNA levels were similar in these two meningioma cell lines and IMR90 normal human fibroblasts, MDM2 and MDM4 protein levels were higher in IOMM-Lee cells than in the other two cell lines (Figure 1A), suggesting that some post-transcriptional mechanism of upregulating the negative regulators of p53 was operative to inactivate the p53 pathway in IOMM-Lee cells.