1. A process for producing a polynucleotide encoding a restriction endonuclease with an altered specificity, which process comprises: (a) mutagenising a polynucleotide encoding a restriction endonuclease with specificity for a recognition sequence so as to produce one or more mutated polynucleotides; and (b) isolating therefrom a polynucleotide encoding a mutated restriction endonuclease with specificity for an altered recognition sequence by selecting a polynucleotide which expresses a restriction endonuclease with methylase specificity for the altered recognition sequence.