Schematic structure of eight NR1 receptor isoforms (NR1A–H). Exons 5, 21 and 22 encode three splice cassettes named N1, C1 and C2. Carboxy-terminals variants are generated by splicing out of cassettes C1 and/or C2; and amino-terminal variant, by splicing out of N1. If C2 is excised, the first stop codon is suppressed, resulting in a new open reading frame that encodes the sequence named C2’ [143].