Epistatic fitness effects have previously been identified in influenza by phylogenetic analyses of HA and NA sequence data (Kryazhimskiy et al. 2011), and from the introduction of mutations into reconstructed historical sequences of the nucleoprotein gene (Gong et al. 2013); cases where mutations have background-dependent effects upon phenotype have been listed elsewhere (Lipsitch and Galvani 2014). Here, XK is linked to influenza.