May play a role in the last steps of the chondrocyte differentiation pathway as an inducer of maturation (PubMed:13679380). Induces chondrocyte calcification during endochondral ossification by playing a role in the transcriptional inhibition of ENPP1, a generator of pyrophosphate which inhibits calcification (PubMed:16680148). Possibly impairs the binding of a transcription factor to the ENPP1 promoter (PubMed:16680148). Unlike other cystatins, does not have thiol protease inhibitor activity (PubMed:16680148). In cartilage, expressed mainly in mature chondrocytes including prehypertrophic and hypertrophic cells (at protein level) (PubMed:13679380). Expressed exclusively in cartilage (PubMed:11856874). In maturing chondrocytes, expression appears at day 3 and increases thereafter. By high phosphate diet. No visible phenotype. Microscopic decrease in the calcification of hypertrophic chondrocytes at the growth plate. Belongs to the cystatin family.