As shown in Table 2, the stratified analysis by histology demonstrated that high expression of CXCR4 predicted unfavorable OS in both NSCLC (HR 1.59, 95% CI 1.40–1.81, P < 0.001, I2 = 43.2%) and SCLC patients (HR 1.77, 95% CI 1.00–3.12, P = 0.050, I2 = 0). This evidence concerns the gene CXCR4 and small cell lung carcinoma.