MDM4 and cancer: For instance, most breast cancer TP53-phenocopied tumors derive from previously known events i.e. the MDM4/MDM2/PPM1D amplifications are the most common event, while the remaining 27% of the TP53 loss phenocopies (5% of all breast cancer samples) are not associated with a known phenocopying event (proportion shown for every cancer type Fig. 1d).