Probable phosphoinositide-binding protein involved in protein sorting and membrane trafficking in endosomes. Plays a role in cilium biogenesis through regulation of the transport and the localization of proteins to the cilium. Required for the localization to the cilium of V-ATPase subunit ATP6V1D and ATP6V0D1, and RAB8A. Involved in osteoclast differentiation and therefore bone resorption. Interacts with ATP6V1D; may play a role in ciliogenesis. May also localize to nucleus and endoplasmic reticulum. Expressed in femur, calvariae and teeth. Strongly up-regulated during osteoclastogenesis. Expressed in calvariae and developing teeth as early as 16.5 dpc (at protein level). The PX domain mediates interaction with membranes enriched in phosphatidylinositol 3-phosphate. Belongs to the sorting nexin family.