Eight studies illuminated the association of Ang-2 expression with tumor differentiation [28, 29, 31, 38, 40–43]; the combined OR revealed that there was a significant association between Ang-2 expression and tumor differentiation (pooled OR = 0.65, 95 % CI: 0.45–0.94, P = 0.02 and I2 = 21 %; Fig. 3b). This evidence concerns the gene ANGPT2 and neoplasm.