GDNF and colorectal carcinoma: Results of MVMR showed that the causal effect of GDNF (IVW; OR = 1.12; 95% CI: 1.05–1.19; P = 2.72 × 10-4) and TRAIL (IVW; OR = 0.93; 95% CI: 0.91–0.99; P = 1.00 × 10-2) on CRC risk remained statistically significant.