The protein, eCD4-Ig acts to inhibit HIV infection. It is a synthetic biologic therapeutic designed for the purpose of preventing HIV from infecting human cells. DNA that encodes for the protein could be delivered, produce the anti-HIV protein, and effectively act as an HIV-1 vaccine. In February of 2015 a team of scientists published data demonstrating the efficacy of this protein in preventing HIV infection.1

eCD4-IG was designed by fusing 4 separate functional protein domains into a single novel protein. The four components include:
1) CD5 Leader is an export signal to get the protein outside the cell.
2) A portion of the extracellular domain of CD4 - a decoy for HIV’s normal attack mechanism.
3) Structural features from the human antibody IgG1 .
4) A peptide, E51 Fragment , known to bind to HIV’s gp120 protein - the lock that prevents HIV from escaping once it has bound the decoy.

Diagram of the four main features of eCD4-IG

Together, these four protein domains form a potent anti-HIV therapeutic. The first domain is a technical requirement to get the protein to where it is needed - outside of the cell. The second domain is an exact duplicate of a portion of the receptor already found on human T-cells. This receptor is how HIV recognizes human T-cells - and upon sensing the receptor, launches its attack on the cell. By creating a mimic HIV will prematurely and ineffectively launch attacks on this protein rather than human T-cells. The third domain is a standard antibody hinge and constant region that signal your immune system to attack. And the final domain is a sequence from an antibody that is known to bind to HIV’s gp120 protein - one of the proteins that does HIV’s attacking. When used in cooperation like this the four domains are able to get the protein outside the cell, attract HIV particles that would otherwise attack cells, trap them tightly, and signal to your own immune system that the virus should be destroyed.

The decoy domain is an exact sequence duplicate of the receptor HIV uses for infection, and the binding peptide binds to machinery that is absolutely required for infection. As such, were HIV to mutate away from being duped and hooked by these two domains, it would simultaneously mutate away its own infectiousness.