|MadSci Network: Molecular Biology|
GAPDH (aka glyceraldehyde-3-phosphate dehydrogenase) is one of a number of so called housekeeping genes. These genes code for proteins that are expressed at a relatively constant rate, both hypothetically (as they are thought to be required for cell viability) and experimentally (they tend not to vary greatly in amount). The value of a protein (or RNA) that is present at a constant level is that it provides a reference against which to compare your protein (or RNA) of interest. For instance, one dish of cells is treated with a drug, and an identical dish is left untreated. Using an antibody to detect protein X by Western blot, it appears that drug treatment increases protein X levels 3 fold. But is this a general increase in protein level? Or is it some sort of experimental artifact? The experiment is repeated using an antibody to GAPDH; no difference is seen between treated and untreated cells. Taken together, the results suggest that there is a specific effect of the drug on Protein X levels. In the ideal case, one would always compare test protein levels against a control present at a constant amount. In practice, nothing turns out to be absolutely constant, and many people normalize to loading equal amounts of total protein.
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