Abstract: Novel methods and reagents for determining the presence of specific nucleic acid base sequences by employing crosslinking reactions of unique molecules capable of forming covalent bonds which are bonded with various labels or ligands for amplication.Single stranded nucleic acid probes are employed which contain complementary base sequences to nucleic acid target molecules. By first hybridizing and then forming covalent bonds between the probe and target the amount of label in the crosslinked hybrid can be measured as an extremely sensitive method for assaying for specific nucleic acid sequences.