The ranking of the six examined genes analyzed by geNorm is shown in Table 3 . In six tissues (muscle, liver, gills, head kidney, spleen and thymus), the EF1A B gene emerged as the most stable, whereas the EF1A A gene was ranked number one in brain and the β-actin gene was ranked number one in intestine. The 18S rRNA and S20 genes were ranked among the worse genes in all tissues. Not surprisingly, the GAPDH gene was ranked worse in five tissues (liver, head kidney, spleen, brain and thymus), confirming the general skepticism against the use of this gene as reference [7, 16, 20] . Combined, the total ranking reads EF1A B >EF1A A >β-actin>18S rRNA>S20>GAPDH. We did not analyze our data with the Bestkeeper software. Analyzing reference genes in virus infected cells, Radonic et al. [4] concluded that the Bestkeeper tool gave results that slightly deviated from, but nevertheless corresponded to, those obtained using geNorm.