The cystatin superfamily is a well-established family of cysteine protease inhibitors. All true cystatins inhibit cysteine peptidases of the papain family, such as cathepsins, while some also inhibit legumain family enzymes. CST9L (Cystatin-9-like) is a 147 amino acid secreted protein that is related to cystatin 9, which is thought to play a role in hematopoietic differentiation or inflammation. The gene encoding CST9L maps to human chromosome 20, which comprises approximately 2 % of the human genome and contains nearly 63 million bases that encode over 600 genes, some of which are associated with Creutzfeldt-Jakob disease, amyotrophic lateral sclerosis, spinal muscular atrophy, ring chromosome 20 epilepsy syndrome and Alagille syndrome. Additionally, chromosome 20 contains a region with numerous genes which are thought important for seminal production and may be potential targets for male contraception.