EIF2AK3 and esophageal squamous cell carcinoma: However, we found that although treating ESCC cells with GSK2606414—a cell-permeable inhibitor of PERK (protein kinase R-like ER kinase), a sensor of ER stress—also had no effect on cell viability, it increased the effects of SP2509 and UNC0642 with respect to reducing cell viability (Figure S7d).