GRIK1 and attention deficit-hyperactivity disorder: Statistically significant lower expression (t = 4.40, P = 0.0001) of GRIK1 mRNA was detected in the peripheral blood of the ADHD probands as compared to the age-matched control (Control Mean ΔCT = 5.90 ± 0.46; Proband Mean ΔCT = 8.67 ± 0.42) (Fig. 2a).