OC90 (otoconin 90) is the major protein component of otoconia, calcium carbonate biominerals essential for gravity sensing and balance in the vestibular system 1. The protein contains two domains homologous to secretory phospholipase A2 and is specifically expressed in the developing otocyst 1. OC90 functions as a scaffold for otoconia biomineralization by sequestering calcium and facilitating crystal nucleation while inhibiting crystal growth in a concentration-dependent manner 2. It induces morphological changes characteristic of native otoconia and modulates calcite crystal growth through formation of anionic surface clusters upon protein folding 2. OC90 works cooperatively with other otoconial proteins like NOX3, with double transfectants showing enhanced calcification compared to single protein expression 3. The protein's expression is regulated by estrogen receptors and vitamin D receptor (VDR), with downregulation associated with otolith shedding and benign paroxysmal positional vertigo (BPPV) 45. While OC90 levels show some elevation in BPPV patients, otolin-1 demonstrates stronger diagnostic potential as a biomarker 6. Interestingly, ectopic OC90 expression has been identified in aggressive cancers like triple-negative breast cancer, where it promotes metastatic functions 7.