Abstract: The invention is a method for detecting a reaction product which signals the presence of a reaction product inducer such as an enzyme. The method enables the recognition of epitopes that form the basis of a detection strategy without the need for specific antibodies to the epitope. In the method, a directly or indirectly labeled modular domain and a biotinylated form of the cognate peptide ligand are used as the basis for a measurable interaction. The peptide ligand can be masked by modifications through, for example, phosphorylation of the Ser or Thr residue, or extension of the amino acid sequence beyond the C-terminal Val. Because the masked residues are critical to binding of the labeled modular domain, masking of at least one of the residues prevents binding. Upon treatment of the masked residue by the appropriate enzyme, (e.g.