From the patterns of competition between amino acids for influx across the blood–brain barrier it appeared that there were four separate systems of transport (see e.g. [43, 44]).System L primarily for neutral amino acids, which can be inhibited by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH);
System ASC primarily for neutral amino acids, which is not inhibited by BCH;
System y+ (sometimes called system Lys+) primarily for basic amino acids;
System N primarily for the nitrogen-rich amino acids glutamine, histidine and asparagine.