In this method, the nuclease BAL 31 is used to make uni- or bidirectional deletions in a segment of cloned DNA. BAL 31 is a complex enzyme and tends to digest a population of double-stranded DNA targets in an asynchronous fashion, Deletions created by BAL 31 are therefore far more heterogeneous in size than those created by processive enzymes such as exonuclease III. - [Read Generation of Bidirectional Sets of Deletion Mutants by Digestion with BAL 31 Nuclease Protocol]
The double-stranded DNA of recombinant plasmid, phagemid, or bacteriophage M13 replicative form DNA is digested with two restriction enzymes whose sites of cleavage both lie between one end of the target DNA and the binding site for universal primer. The enzyme that cleaves nearer the target sequence must generate either a blunt end or a recessed 3' terminus; the other enzyme must generate a four-nucleotide protruding 3' terminus. - [Read Generation of Sets of Nested Deletion Mutants with Exonuclease III Protocol]