Methods for labelling nucleic acids

The invention relates to methods for labelling individual nucleic acid molecules present in a sample, comprising contacting the nucleic acid molecules with an adaptor or mixture of adaptors, wherein the adaptor or adaptors comprise one or more universal nucleotide bases and a ligation moiety at their 3′ end, and ligating an adaptor to the nucleic acid of interest, wherein the adaptor is ligated to the nucleic acid molecules at the 3′ end of the adaptor. A random tag is then generated in situ by conducting an extension reaction over the ligated adaptor. Methods of the invention may be used to detect genetic alterations or variants in any nucleic acid with high specificity and high sensitivity, including mutations in nucleic acids such as ctDNA, cfDNA, and in viral, microbiome and plant nucleic acids. Methods of the invention may also be used in detection and correction of errors introduced into nucleic acids during processing.