CST11 is a secreted member of the cystatin family. Its annotated function is antibacterial activity against the Gram-negative bacterium E. coli, and it may play a role in sperm maturation and fertilisation. The NCBI Gene summary describes it as an epididymal-specific protein with demonstrated antimicrobial activity against E. coli, which places it at the interface between reproductive biology and innate host defence.

The CST11 gene lies in the cystatin locus on chromosome 20, which contains the majority of the type 2 cystatin genes and pseudogenes. HGNC classifies it in the type 2 cystatin gene group. According to RefSeq, alternative splicing of the CST11 gene yields two variants that encode distinct isoforms. Mouse and rat each have a Cst11 ortholog.

The cystatin superfamily groups proteins that contain multiple cystatin-like sequences. Some members are active cysteine protease inhibitors, while others have lost, or perhaps never acquired, this inhibitory activity. The superfamily comprises three inhibitory families: the type 1 cystatins (stefins), the type 2 cystatins and the kininogens. Type 2 cystatins are cysteine proteinase inhibitors found in a variety of human fluids and secretions. CST11 is also recorded under the names dJ322G13.6, CTES2 and CST8L, which is useful to know when searching the literature for the CST11 protein.