A schematic diagram of the constructions used in this study and the alignment results for the expressed PiCV capsid gene. (A) The full-length wild-type and codon-optimized PiCV capsid protein genes were cloned independently into three expression vectors pET28a, pGEX-4T-1 or pET32a. The PiCV capsid protein with the various different fusion tags, namely a six-histidine (6xHis), a Glutathione-S-transferase (GST) and a Thioredoxin-coupled six-histidine (Trx) at its N-terminus were expressed by T7 or Tac promoter-driving after IPTG induction. (B) The nucleotide sequences were compared between the wild-type (WT) and the codon-optimized (OPT) PiCV capsid protein genes. The asterisk (*) represents the fact that the aligned nucleotides are identical.