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Cathepsin B is a very complex protein which is a part of the super family of papain-like cysteine proteases.  These papain-like cysteine proteases are abundant in nature and can be found in baculovirus, eubacteria, yeast, plants, and animals.  They are synthesized as inactive pro-enzymes with N-terminal pro-peptide regions.  The pro-peptide plays an important role as an inhibitor of enzymatic activity.  A number of cysteine proteases are located within lysosomes and they have very specific functions in endo and exopeptidase activity.  Cathepsin B is synthesized as a pre-proenzyme of 339 amino acid residues and it can act as an endopeptidase.  Its activity is inhibited by a2-macroglobulin.  The most crucial functions of Cathepsin B are that it is implicated in the turnover of proteins and carries out different roles in maintaining the normal metabolism of cells.  Cathepsin B is found in humans and other mammals and it is encoded by a single gene and this gene exhibits a high degree of sequence homology to other cysteine proteases of the papain super family.  However, in parasitic helmiths and free-living nematodes, the cathepsin B genes can be seen as large multigene families instead of just one single gene shown in humans and mammals.  The function of this cathepsin B gene has not been verified for sure, but research is showing that cathepsin B could be involved in induction of protective immunity. In diseases, such as cancer and arthritis, Cathepsin B gene shows up regulation and in Alzheimer’s disease down regulation is shown.