ID1 and esophageal squamous cell carcinoma: Id1 overexpression shows not only high correlation with tumor invasion, metastasis and poor prognosis in esophageal squamous cell carcinoma (ESCC), but it also plays a crucial role in the resistance to the anticancer drugs etoposide [301] and 5-fluorouracil (5-FU) [302]: indeed, etoposide enhances c-Jun/c-Fos expression that leads to Id1 gene transcription and expression, resulting in inhibition of apoptosis [301].