Secretion of Fib₄₂₀ by transfected COS cells in the presence and absence of the common  chain. Cells were transfected with pED4–Neo-vectors containing fibrinogen subunit cDNAs in combination as indicated below each lane; _E cDNA was included at a fivefold excess. Fibrinogen was immunoprecipitated from culture medium with either antifibrinogen (f) or anti-VI (VI) as indicated and run under nonreducing conditions. Traces of incompletely assembled or single subunits could be seen in the lower part of the gel only after overexposure of the film, confirming that most of the chains detected in the medium upon reduction of the immunoprecipitates in Fig1B were indeed components of hexameric fibrinogen molecules.