The rate at which mutations occur over unit time. It can be expressed in various ways. For example, the ‘time’ units may be per replication or cell division, per gamete, or per generation; moreover, the number of mutations can be expressed as an average probability for each mutable site (i.e. per nucleotide), or as a total for the entire genome. Thus, the spontaneous mutation rate for a wild-type strain of the bacterium E. coli has been calculated as 1 × 10^–3/genome/generation, based on the mutations accumulated over many generations. Estimates of the mutation rate in humans are generally in the range of 100–160/diploid genome/generation, depending on the method of calculation. In terms of genomic DNA, this equates to a mutation rate of the order of 10^–9/base pair/year; similar mutation rates have been found for other organisms. Compare mutation frequency.