Since gp120 plays a vital role in the ability of HIV-1 to enter CD4+ cells, its evolution is of particular interest. Many neutralizing antibodies bind to sites located in variable regions of gp120, so mutations in these regions will be selected for strongly. The diversity of "env" has been shown to increase by 1-2% per year in HIV-1 group M and the variable units are notable for rapid changes in amino acid sequence length. Increases in gp120 variability result in significantly elevated levels of viral replication, indicating an increase in viral fitness in individuals infected by diverse HIV-1 variants. Further studies have shown that variability in potential N-linked glycosylation sites (PNGSs) also result in increased viral fitness. PNGSs allow for the binding of long-chain carbohydrates to the high variability regions of gp120, so the authors hypothesize that the number of PNGSs in "env" might affect the fitness of the virus by providing more or less sensitivity to neutralizing antibodies. The presence of large carbohydrate chains extending from gp120 might obscure possible antibody binding sites.