It would be expected that measuring expression of an intact mRNA would yield approximately equal results regardless of the region being probed, and if mRNA fragmentation had occurred, then some sequences may be more abundant than others. We thus tested the effect of PCR probe location on the RNAs. The 5 0 and 3 0 GUSB probes, separated by 1209 nt, were associated with highly similar threshold cycle (Ct) measures (r = 0.98, b parameter = 0.88) ( Figure 5C ). Similar results were obtained for TFRC, with probes separated by 2066 nt (r = 0.84, b parameter = 0.92, data not shown). It seems therefore that the region being probed is not a source of variation in our results.