Presence or emergence of different HIV subtypes may also account for the appearance of antiretroviral drug resistance as well as the variability in pathologic lesions as different cell types are targeted or different cytopathic effects are elicited during the course of infection. Phylogenetic studies can identify genetic clusters of HIV-1 env genes which are known as subtypes, or clades, that have arisen with progression of the AIDS epidemic worldwide. The V3 loop amino acid sequences of these genetic variants influence HIV phenotype and immune response. Thus, the biologic properties of HIV can vary, even within an individual HIV infected person, where variants of HIV may arise that are "neurotropic" or "lymphocytotropic" for example.