PAK1 and esophageal squamous cell carcinoma: It has been demonstrated that The Raf1/MEK1/ERK cascade plays a central role in ESCC biology [32–34], and PAK1 regulate ERK activity in some cell types by phosphorylating Raf1 at S338 and MEK1 at S298 [35–37], which suggests that the effect of PAK1 on ESCC cells involves the Raf1/MEK/ERK pathway.