The inclusion of the third housekeeping gene (L23), in addition to F-BOX and PP2A, may have reduced the V value to below the cut-off threshold of 0.15, which would have suggested that the top three reference genes (F-BOX, PP2A and L23) would be adequate in our qRT–PCR normalisation of different viral infections, and an additional reference gene was not required. The gene discussed is PTPA; the disease is viral infectious disease.