Genes encoding antirestriction proteins are found in transmissble plasmids (ardABC) and bacteriophage genomes (ocr, darA). Antirestriction proteins inhibit type I restriction-modification enzymes and thus protect the unmodified plasmid or phage DNA from degradation. Antirestriction proteins belong to the family of DNA-mimicry proteins, whose spatial structure mimics the B-form of DNA. Based on an analysis of the mutant forms of ArdA and Ocr obtained by site-directed mutagenesis and the native form of ArdA that specifically inhibit type I restriction enzymes but do not affect their methylase activity, a model is proposed to describe the complex formation between an antirestriction protein and a type I restriction-modification enzyme (R₂M₂S): antirestriction proteins can displace a DNA strand from its binding sites in the S subunit (which contacts a specific site on DNA) and in the R subunit (which translocates the DNA strand and cleaves it). Antirestriction and antimodification activities of ArdA and Ocr as a function of ardA and ocr expression levels were studied by cloning the genes under a strictly regulated promoter.