A specific region of the <taxon tax_id="11520">Influenza B virus</taxon> NS1 protein, which includes part of its effector domain, blocks the covalent linkage of mouse ISG15 to its target proteins both <i>in vitro</i> and in infected cells. Of the several hundred proteins induced by interferon (IFN) alpha/beta, the ubiquitin-like ISG15 protein is one of the most predominant. <taxon tax_id="11320">Influenza A virus</taxon> employs a different strategy: its NS1 protein does not bind the ISG15 protein, but little or no ISG15 protein is produced during infection [<cite idref="PUB00007421"/>].