Pepstatin A (NSC-272671; NSC272671), a pentapeptide, is a potent and selective aspartic protease inhibitor which also inhibits HIV replication. Pepstatin A was originally isolated from the microbe. For the enzymes hemoglobin-pepsin, hemoglobin-proctase, casein-pepsin, casein-proctase, casein-acid protease, and hemoglobin-acid protease, respectively, its IC50 values are 4.5 nM, 6.2 nM, 150 nM, 290 nM, 520 nM, and 260 nM. At weak acid pH values, pepstatinA, a highly potent renin inhibitor, inhibited both human and porcine renin with IC50 values of 15 μM and 0.32, respectively. For aspartic proteinases like pepsin, cathepsins D and E, pepstatin A effectively inhibits them. Pepstatin A appears to have no harmful effects on HIV-infected cells, but it does block a portion of the intracellular processing of the HIV gag protein in H9 cells that have the virus. By preventing ERK signaling and reducing NFATc1 expression, pepstatin A also prevents osteoclast differentiation.