The present invention relates to a method of labelling nucleic acid using oligonucleotide-peptide conjugates to hybridise to nucleic acid. The invention also relates to methods of production of oligonucleotide-peptide conjugates, and to nucleotide intermediates used in the generation of these conjugates that posses mercaptoethyl groups protected by tert-butylthio groups. These intermediates can form oligonucleotide-peptide conjugates through reaction with a maleimido-peptide.