CRISPR/Cas9 endonuclease-mediated genome editing is used to create an amino acid substitution at position 544 (proline to leucine, P544L, CCA to CTG) in the 635 amino acid (aa) isoform 2 (Ensembl transcript Slc6a8-203). The 635 aa murine isoform 2 is syntenic to the 635 aa human isoform 1. (J:101977)