The four major classes of protease enzymes 1., 2., 3. and 4. (aspartic, serine, cysteine and metallo) selectively catalyze the hydrolysis of polypeptide bonds. Proteases of these classes are also crucial for disease propagation, and inhibitors of such proteases are emerging with promising therapeutic uses. 3. and 5. Among them, serine protease inhibitors are useful for the treatment of diseases like cancer, 6., 7. and 8. viral infections (eg HIV, 9., 10. and 11. ...