The gene known as HCP5 (HLA Complex P5) is a human endogenous retrovirus, meaning that it is a fossil of an ancient virus that at one time infected people, but has now become an integral part of the human genome.
One variation of HCP5 appears to provide some delay or resistance to the development of AIDS when a person is infected with HIV. This variation of HCP5 frequently occurs in conjunction with a particular version of an immune system gene called HLA-B.
References
HCP5 Wikipedia(Text) CC BY-SA