Measurements of mRNA expression levels—whether by Northern analysis, ribonuclease protection, or real-time quantitative PCR—are usually standardized by comparing the data to that obtained for an internal or endogenous reference gene. Housekeeping genes such as beta-actin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are most often used because their expression levels are expected to remain constant under different treatment conditions. Unfortunately, this assumption is not always valid, and results based on housekeeping genes alone can be biased (Suzuki, Higgins, and Crawford 2000). A better method is to normalize your data using our qPCR Human Reference cDNA, the only cDNA control derived entirely from human tissues.