Since oligo dT starts at the 3 prime end, if ur Realtime primers are not towards the second half of the mRNA, some long transcripts cannot be quantitatively analyzed (one friend of mine struggled with Realtime for long time due to this unexpected problem) .
Is a random primer or oligo(dT) better for RT-PCR reaction? - ResearchGate. Available from: https://www.researchgate.net/post/Is_a_random_primer_or_oligodT_better_for_RT-PCR_reaction [accessed Oct 6, 2015].