The present invention provides methods of allele-specific primer extension useful for detecting mutations and genetic variations. In particular the invention provides a method of detecting a base at a pre-determined position in a nucleic acid molecule, said method comprising: performing primer extension reactions using base-specific detection primers, each primer being specific for a particular base at said predetermined position, and comparing said primer extensions to determine which base is present at said position, wherein said primer extension reactions are performed using labelled nucleotides and wherein a nucleotide-degrading enzyme is present during the primer extension reaction.