Table 1. Overview of the different stoichiometric parameters and the mutations in the envelope protein, that might be used to study these parameters.

parameter	Definition	mutation
CD4 subunit stoichiometry	number of envelope protein – CD4 interactions within one trimer required for viral entry	envelope deficient of CD4 binding, e.g. [19]
coreceptor subunit stoichiometry	number of envelope protein – coreceptor interactions within one trimer for viral entry	envelope deficient of coreceptor binding, e.g. [19]
fusion subunit stoichiometry	number of fusion proteins per trimer that have to be exposed for membrane fusion	envelope with a non-functional fusion protein unit, e.g. [19]
subunit cooperation	Do two functions A and B (e.g. receptor binding or revelation of the fusion domain) have to be located on two different subunits?	(i) envelope proteins defective of the two functions simultaneously; (ii) two different mutants: each defective of one of the two functions