Some restriction endonucleases , cleave DNA at random, but a particular group of enzymes, known as class II restriction endonuclearses, cleave DNA at special sits.

• 一些限制性内切酶的切割DNA的作用是随机的，但是相当一部分的内切酶，如限制性内切酶II，就是在特定的位置切割DNA。