The GAD gene (Genbank: M84025) was amplified from E. coli genomic DNA by a polymerase chain reaction (PCR) using a forward primer (5'-GAAGGAGATATACATATGGATAAGAAGCAAGTAACG-3') and a reverse primer (5'-GTGGTGGTGGTGGTGGTGCTCGAGGGTATGTTTAAAGCT-3'), where the underlined base pairs were introduced for in-fusion ligation.