The Klotho proteins, Klotho and Klotho beta (beta KL), are type I transmembrane glycoproteins with extracellular regions that contain two beta-glycosidase-like repeats. Klotho has both membrane-bound and soluble forms; the soluble form predominates and consists of only the first beta-glycosidase-like repeat. Klotho has beta-glucuronidase activity against select steroid beta-glucuronides, appears to regulate calcium and vitamin D levels in serum, and is associated with premature aging.