PCR is particularly valuable for monitoring gene expression, quantifying food-borne pathogens, testing viral load, and also for clinical diagnosis. However, although it can be extremely effective with pure solutions of nucleic acids, its usefulness is limited, in part, by the presence of inhibitory substances. Originating from the samples or from DNA extraction protocols, these can reduce or even block amplification. Although many biological samples have been reported to inhibit PCR amplification, the biochemical and physical mechanisms and identities of many inhibitors remain unclear. © 2004 by CRC Press LLC.