CD40 and attention deficit-hyperactivity disorder: The 2-step MR analysis suggested that CD40 levels may exert an indirect effect on ADHD via N-acetylneuraminate, although the mediation effect did not reach statistical significance (mediation effect: 0.005; 95% CI, -0.000 to 0.011; mediation proportion: -7.290%; 95% CI, -15.100% to 0.506%).