ATF4 and neoplasm: Similarly, Condamine et al. (2014) revealed that tumor-infiltrating MDSCs ex vivo exhibited upregulated expression of ER stress response genes, including CHOP, XBP-1, BIP and ATF4. However, the PERK/ATF4/CHOP axis will initiate cancer cell apoptosis if ER stress is excessively severe and unsettled (Condamine et al., 2014).