CST9 belongs to the CRES （cystatin-related epididymal spermatogenic） subfamily. The CRES （cystatin-related epididymal spermatogenic） protein outlines a new subgroup in the family 2 cystatins of the cystatin superfamily. A few members are active cysteine protease inhibitors， however others have lost or possibly never developed this inhibitory activity. CST9 takes part in hematopoietic differentiation or inflammation and is expressed in heart， placenta， lung， liver， skeletal muscle and pancreas. CST9 is upregulated by LPS in several cancer cell lines， such as promyelocytic leukemia （HL-60） and myelomonocytic leukemia.
