GRM6 and attention deficit-hyperactivity disorder: Statistically significant lower expressions of GRM5, GRM6, GRM7, GRIA1, GRIN2A, and GRIN2B mRNA were detected in the ADHD probands as compared to the age-matched controls (Fig. 4A; data expressed as ΔCT mean ± SEM; Supplementary Table S5; P < 0.0001).