A nucleic acid molecule is provided which is initially catalytically inactive but which can complex with a specific co-factor, e.g., a nucleic acid molecule or a non-nucleic acid molecule, to form a catalytically active nucleic acid molecule. The catalytically active nucleic acid molecule can be used to detect the presence of a non-nucleic acid co-factor or of a nucleic acid co-factor using the nucleic acid sequence of the present invention.