At 24 to 48 hours after injury, tau levels in athletes with concussion were significantly lower than baseline levels (mean difference, −0.320 pg/mL; 95% CI, −0.461 to −0.178 pg/mL; P < .001) and levels in both control groups (contact sport controls: mean difference, −0.285 pg/mL; 95% CI, −0.475 to −0.094 pg/mL; P = .001; non–contact sport controls: mean difference, −0.217 pg/mL; 95% CI, −0.425 to −0.009 pg/mL; P = .04). The gene discussed is MAPT; the disease is concussion.