MDM4 and breast cancer: Our findings demonstrate, however, that some of the genes amplified only in the HER2-negative components of HER2 heterogeneous breast cancers, such as genes previously described as oncogenic, including MDM4 [32], FGFR1 [40,46], ZNF703 [33], MYC, FAM83A [34], RAB11FIP1 [28,29], and PIK3CA (Table 1), as well as BRF2 and DSN1, which were shown to have oncogenic properties here, may constitute potential drivers and compensate for the lack of HER2 amplification.