The RT-PCR method can be used not only to detect specific mRNAs but also to semi-quantitate their levels. Thus, one can compare levels of transcripts in different samples. This can be done in two different ways. One is to quantitate against levels of transcripts from a control, house-keeping gene (such as actin and GAPDH). (Transcription of house-keeping genes is believed to be unaffected by almost all experimental conditions.) The second method is to add an exogenous, primer-specific PCR template during PCR.
Semi-quantitation using exogenous standards
One can have exogenous standards (either RNA or DNA) that can be amplified using the same primers that amplify the transcript of interest. The standards are designed to have the same sequence as that of the transcript of interest except that they are 40-50 nucleotides shorter or longer. Different amounts of the standard is used in different PCR reactions (if the standard is an RNA, it is added prior to the RT step) and amplified for same number of cycles. The exogenous standard competes with the endogenous template, thus this strategy is also refered to as competitive RTPCR.
Products are electophoresed and quantitated by software and an 'amount of standard' vs 'band intensity' plot is drawn.