This is an RNA or DNA that is naturally present in each experimental sample. By using an invariant endogenous control as an active ‘reference’, quantitation of a messenger RNA (mRNA) target can be normalized for differences in the amount of total RNA added to each reaction and correct for sample-to-sample variations in reverse transcriptase PCR efficiency. See ABI TaqMan Human Endogenous Control Plate; TATAA Biocenter Endogenous Control Gene Panel; geNorm kit; Ambion: 18S RNA as an Internal Control; Ambion: GAPDH, b-actin, cyclophilin, 18S RNA as internal controls; EXPOLDB: The most constantly expressed housekeeping genes; algorithms to select the best endogenous controls: geNORM (Vandesompele, 2002), NormFinder (Andersen, 2004), and qBasePlus (Hellemans, 2007).