Human-adapted bacterial pathogens use a mechanism called phase variation to randomly switch the expression of individual genes to generate a phenotypically diverse population to adapt to challenges within and between human hosts. There are increasing reports of restriction-modification systems that exhibit phase-variable expression. The outcome of phase variation of these systems is global changes in DNA methylation. Analysis of phase-variable Type I and Type III restriction-modification systems in multiple human-adapted bacterial pathogens has demonstrated that global changes in methylation regulate the expression of multiple genes. These systems are called phasevarions (phase-variable regulons). Phasevarion switching alters virulence phenotypes and facilitates evasion of host immune responses. This review describes the characteristics of phasevarions and implications for pathogenesis and immune evasion. We present and discuss examples of phasevarion systems in the major human pathogens Haemophilus influenzae, Neisseria meningitidis, Neisseria gonorrhoeae, Helicobacter pylori, Moraxella catarrhalis, and Streptococcus pneumoniae.

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