Klotho is an aging regulator and pleiotropic hormone primarily produced in the kidney and choroid plexus of the brain that circulates throughout the body following cleavage from its transmembrane form.1, 2, 3, 4, 5, 6 Well studied in model organisms (for example, mice4 and nematodes7), klotho is an aging regulator that, when overexpressed, extends lifespan4, 7 and, when disrupted, promotes aging phenotypes,3 including atherosclerosis,8 decreased bone mineral density6, 9, 10 and osteopenia.11 The effects of klotho are complex and multifaceted. The gene discussed is KL; the disease is Osteopenia.