CD4 and infection: Since SOSIPs are GP140 proteins without a transmembrane domain, it should be taken into account that when using virus-infected cells, binding of bNAbs to SOSIPs could be due to released GP140 that binds back via CD4 or lectin receptors; it could also be due to some membrane retention during infection as part of Env binding with membrane components of the virus (in fact when MVA is purified from chick cells, SOSIPs are present in the membrane fraction of the virion; data not shown).