Limited influenza mutants have been examined for fitness costs and drug resistance.

Fitness effects in critical regions of N1 NA were quantified by EMPIRIC (Exceedingly Meticulous and Parallel Investigation of Randomized Individual Codons).

Many mutations exhibited increased fitness in the presence of oseltamivir.

NA position 223 is a hotspot for mutations that decrease binding to oseltamivir.

Drug resistance results from a balance of drug binding and substrate processing.