On the basis that the only behaviour regulated by diversification in both GluN2A and GluN2B was impulsivity, and that anxiety and motor activity required a unique amino acid sequence from either the GluN2A or GluN2B CTD, they also suggested that the protein sequences controlling emotional and motivational behaviour were less constrained by natural selection than those sequences that regulate learning behaviours, as three of the four learning behaviours did not require an amino acid sequence unique to either GluN2A or GluN2B. The gene discussed is GRIN2B; the disease is Anxiety.