Cover. Functional evolutionary hierarchy of restriction-modification systems. Enzyme subunit organization for each type of system is shown on the left. The two closely marked vertical lines constitute a recognition site. The distance between the recognition and cleavage sites is shown in base pairs. A DNA molecule shown in the background is to commemorate the 50th anniversary of the discovery of double helix. For further details see the paper by Raghavendra and Rao in this issue [Nucleic Acids Res. (2003) 31, 1888--1896].
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