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.