Abstract: The present invention relates to methods for generating recombinant viruses from samples such as uncharacterised virus samples or clinical specimens and to the use of the viruses so generated in assays, predominantly for the purpose of detecting altered viral susceptibility to anti-viral drugs and reagents. The invention further relates to deoxyribonucleic acid (DNA) sequences applied in the new methods and assays. The methods and assays are adapted to detect resistant virus more rapidly, sensitively and accurately than known assays by taking into account additional compensatory mutations arising in nucleotide sequences other than those encoding the anti-viral drug target, and by requiring fewer recombination events to generate recombinant virus.