Studies aimed at identifying the most suitable reference genes in the ovine species have been performed in nervous tissues, spleen, mesenteric lymph node, ileum, lung and pulmonary artery [5] [6] [7] [8] . However, no specific information on whole blood is currently available. A reference gene for use in peripheral blood mononuclear cells was selected [9] , but this study was based on the analysis of the standard deviation of cycle threshold (C t ) and not on specifically designed algorithms. Nevertheless, blood is a readily accessible source of material for analysis and some attempts to identify gene expression markers by qRT-PCR in order to develop blood tests in sheep have been reported for prion diseases. For example, in 2001, Miele et al. discovered a novel erythroid-associated factor (ERAF) and demonstrated a dramatic decrease in expression of the specific transcript within rodent models of prion diseases, providing the first easily detectable molecular marker in a readily accessible tissue [10] . More recently, analysis of blood by qRT-PCR from sheep experimentally infected with scrapie revealed that the extent of differential expression of ERAF in peripheral ovine blood may be insufficient to provide a discriminatory diagnostic test [11] . However, the lack of a set of validated reference genes for sheep whole blood did not allow for the proper normalization of gene expression data and, in this study, glycophorin C (GYPC) was arbitrarily chosen as normalizer based on its higher expression in the human erythroid lineage. Moreover, the use of a single gene to normalise expression is no longer considered sufficient [12] [13] [14] [15] . Vandesompele et al. (2002) demonstrated that errors of up to 20-fold in expression data can be generated by the use of only a single reference gene [1] .