Genetic mutations in HIV are well known and are very likely, considering the presence of two RNA molecules per virus. Either or both RNA molecules can mutate. These mutations potentially lead to drug resistance or encourage the virus to evade the body's immune response.
Mutations have created three major groups of HIV-1: M, N, and O. Most HIV infections are HIV-1 Group M. The M group is further classified into subtypes (also referred to as clades) and even sub-subtypes.
Another group, Group P, was identified in 2009. However, it remains very rare with only two HIV-1 cases identified as Group P as of 2011.
The image on the right illustrates the classifications of the the major HIV types, groups, and subtypes that have been identified to-date.