Mutation of potential N-glycosylation sites in the _E VI-domain: Determination of carbohydrate attachment site. The VI-domain’s two potential glycosylation sites, at Asn667 and Asn812, were changed to Gln; a new site at Asn791 had been introduced by the Cys793 → Ser change. COS cells were transfected with either wild-type or these mutant _E cDNAs together with the β and γ subunit cDNAs. Fibrinogen was immunoprecipitated from cell lysates and culture medium with antifibrinogen and run under reducing conditions. Upon overexposure of the film, Fib₄₂₀ subunits are clearly detectable in the culture medium of the Q667 mutant.