Adding glycosylation sites to the 1918 H1N1 HA globular head similarly attenuates viral replication in mice, while deletion of HA glycosylation sites N142 and N177 from A/Solomon Islands/2006 increases virulence . Although in this instance the role of the UPR remains unconfirmed, similar phenotypic responses to infection with the H3N2 and H5N1 glycosylation mutants suggest modulation of the UPR is a plausible mechanism. The role of individual glycosylation sites in determining productive protein folding of HA has long been established .