= Solution
A strong specificity control is competition with a large excess of unlabelled cognate promoter <DNA>. It should reduce the shifted labelled complexes, whereas an unrelated or binding-site-mutant competitor should compete much less effectively. \b[This tests sequence-dependent association rather than non-specific probe retention.] A probe-only lane identifies the free probe, and an irrelevant isotype-matched <antibody> controls whether any added <antibody> produces aggregation or altered mobility. These controls address different uncertainties and should not be treated as interchangeable.
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