Rates of substitution mutations in two directions, v [from an A-T or T-A nucleotide pair (AT-pair) to a G-C or C-G nucleotide pair (GC-pair)] and u [from a GC-pair to an AT-pair], are usually not the same. The net effect, v/(u + v), has previously been defined as directional mutation pressure (mu D), which explains the wide interspecific variation and narrow intragenomic heterogeneity of DNA G + C content in bacteria.