For most restriction enzyme
s, this is the short (typically a half-dozen or so base
s long) sequence of DNA
which is recognised and consequently within which the DNA is restricted (cut)
eg. that for EcoRI is GAATTC, with cleavage occurring between the G and A on each strand to leave sticky ends.
For type IIs restriction enzymes, the recognition site and cleavage site are separated by a number of bases characteristic to the particular enzyme.