Cathepsin Z is a lysosomal cysteine proteinase and belongs to the peptidase C1 family. Human Cathepsin Z contains a singnal sequence, a propeptide and a mature chain. It exhibits bothcarboxy-monopeptidase and carboxy-dipeptidase activities. In contrast to cathepsin B, it does not act as an endopeptidase. Cathepsin Z is restricted to the cells of theimmune system, predominantly monocytes, macrophages and dendritic cells. It is expressed ubiquitously in cancer cell lines and primary tumors. Like other members of this family, Cathepsin Z may be involved in tumorigenesis.