Expression levels of three housekeeping genes (HKG)-GAPD, GUSB and TFRC-were measured by quantitative PCR using the TaqMan Gene Expression Assays according to the manufacturer's instructions (Applied Biosystems, USA). Sixteen aliquots of a unique batch of RNA sample (Universal Human Reference RNA, Stratagene, USA) of various levels of integrity (cf. Table 1 ) were used to test the influence of RNA quality on the relative expression of those three genes. In parallel, a 5 0 to 3 0 comparison was done using two separate GUSB and TFRC TaqMan probes.