Previously, the 18S rRNA gene was considered to be an ideal internal control in qRT-PCR analysis (Ambion [15] ). Ribosomal RNA constitutes up to 80-90% of total cellular RNA, and several studies have shown that rRNA varies less under conditions that affect the expression of mRNAs (discussed in Bustin & Nolan [16] ). However, questions have been raised against the use of ribosomal RNA genes as references. Vandesompele et al. [5] have stressed the fact that there sometimes might be imbalances in rRNA and mRNA fractions between different samples, making genes encoding ribosomal RNAs unsuitable as references.