Porcine reproductive and respiratory syndrome virus ORF5a protein is essential for virus viability

► RNA sequences in the overlapping region between ORF5 and ORF5a was essential for virus viability. ► Inactivation of ORF5a expression was lethal for the production of infectious virus. ► Viable PRRSV could be recovered by expressing ORF5a protein in trans. ► ORF5a protein could be putatively extended to 63 aas.