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.