Genetic stability of the rescued mutant viruses. DNASTAR v7.1 program (Lasergene Package) was used to conduct nucleotide sequence alignment of the rescued mutant viruses. Dots indicated that the residues match APRRSV (GQ330474) exactly. All mutant sequences were shown as lowercase. Short lines meant the deletions in D5 mutant viruses comparing with APRRSV. The numbers indicated the sequence positions of APRRSV.