MAPK8 and esophageal squamous cell carcinoma: Our previous study indicated that RAD18 expression is positively associated with tumor invasion, metastatic recurrence, and unfavorable prognoses in patients with ESCC, and that this is attributable to Jun N‐terminal kinase (JNK)‐mediated activation of matrix metalloproteinase (MMP)‐2/MMP‐9.14