HIV-1, the most common HIV strain, is enveloped, spherical, and contains a ssRNA genome of 9.75 kilobases. Upon infection, HIV protease cleaves the gag polyprotein into multiple proteins, including envelope, capsid and regulatory proteins. The envelope protein gp160 further is processed into two key proteins: gp120, used for binding host CD4 receptors, and gp41 for mediating host membrane fusion. The capsid protein p24 is a common antigen for diagnosing HIV infection.