Pepstatin A was originally purified from Actinomycetes species. The peptide is unusual in containing the amino acid statine (4-amino-hydroxy-6-methylheptanoic acid also known as AMHA). Pepstatin A competitively binds with acid proteases in a highly selective reversible manner to inhibit protease activity. Pepstatin A is ineffective on thiol, neutral and serine proteases. The functions of proteases have been investigated by the application of pepstatin A such as renin, pepsin, bovine chymosin and retroviral proteases from HIV. Characterisation of HIV protease using pepstatin A has been vital in development of HIV treatment to block viral replication. Pepstatin A is also a reagent to disrupt autophagy; this helps characterise the function of proteosome degradation in research such as during influenza A replication and improving drug delivery of therapeutic cancer treatments.