DLL4 and squamous cell lung carcinoma: As shown in Fig. 5, a statistically significant association between DLL4 expression and NSCLC overall survival rate was found (NSCLC: pooled HR = 1.60, 95%CI: 1.18-2.17, p = 0.547 and I2 = 0.0%; lung adenocarcinoma: pooled HR = 1.66, 95%CI: 1.10-2.50, p = 0.206 and I2 = 36.7%; lung squamous cell carcinoma: pooled HR = 2.56, 95%CI: 1.56-4.19, p = 0.169 and I2 = 47%).