 Molecular evolutionary studies also revealed the existence of pseudogenes, non-coding stretches of DNA derived by the tandem duplication and subsequent inactivation by mutation of single copy genes. The lack of function of the pseudogene makes all of its codons effectively neutral and provides another estimate of the rate of neutral evolution. Importantly, “replacement” sites that evolve slowly in the functional gene have been shown to evolve more rapidly in the nonfunctional tandem duplicate pseudogene.