Specialized spikes (envelope glycoproteins, Env, colored orange, red, and yellow), stand out on the surface of the human immunodeficiency virus (HIV). The spikes allow HIV to attach to CD4 and CCR5 proteins (blue, purple) on human immune cells. Researchers are developing versions of Env for vaccines, so that the immune system will recognize and combat the real spikes on invading viruses if a person is exposed to HIV. Sanders et al., HIVâ€™s Achille's Heel. Scientific American 315(6): 53.