NormFinder analysis of housekeeping genes in human (a), pig (b), chicken (c) and duck (d) cells showing stability numbers, which are a measure of the systematic error introduced by each of the reference genes when used to normalise the qRT-PCR data. In all of the four cell types, 18S rRNA was the best reference gene as evidenced by the lowest stability numbers (0.003 to 0.016) among all of the genes compared. GAPDH was the second gene of choice for all the four host species. ACTB was the most unstable gene in all four host cell types. Data represent stability numbers which were calculated by combining intra- and inter- group variations (n=12).