CIQ is a substituted tetrahydroisoquinoline that acts as a subunit-selective potentiator of NR2C- and NR2D-containing NMDA receptors. It is without effect at subunits NR2A, NR2B, or glutamate receptors. CIQ enhances receptor responses two-fold (EC50 = 3 µM) by increasing channel opening frequency for glutamate or glycine. It is a positive allosteric modulator that does not alter agonist EC50 values.