CCR5 is the primary co-receptor used by gp120 sequentially with CD4. This bind results in gp41, the other protein product of gp160, to be released from its metastable conformation and insert itself into the membrane of the host cell. Although it has not been confirmed, binding of gp120-CCR5 involves two crucial steps: 1) The tyrosine-sulfated amino terminus of this co-receptor is an "essential determinant" of binding to gp120 (as stated previously) 2) Following step 1., there must be reciprocal action (synergy, intercommunication) between gp120 and the CCR5 transmembrane domains.