A good internal control is critical in all quantitative analyses of gene expression. Levels of bet-actin, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and peptidylprolyl isomerase A (PPIA) were analyzed in 78 samples (data obtained from our laboratory and from a publicly available database at http://www.ncbi.nlm.nih.gov/SAGE/). These libraries included cell lines and tissues from brain, breast, colon, kidney, ovary, pancreas, prostate, skin, and vascular origin. The level of PPIA mRNA is the most constant among the three genes. Hence, our study suggests that PPIA is a better internal control than beta-actin or GAPDH, the two most commonly used internal controls.