α-klotho (α-kl) was first identified as an aging gene and was later shown to be a regulator of mineral homeostasis. α-kl (- / -) mice display multiple aging related phenotypes including atherosclerosis, cardiovascular/soft tissue calcifications, pulmonary emphysema, osteopenia, and senile atrophy of skin ; such age-related organ pathologies are associated with biochemical changes in blood, including severe hyperphosphatemia, elevated serum FGF23 and1,25 (OH) 2 Vitamin D levels. Of significance, advanced stage patients suffering chronic kidney disease (CKD) develop multiple complications quite resembling phenotypes observed in α-kl (- / -) mice, and high serum phosphate, the major cause of abnormalities of α-kl (- / -) mice, has been reported to be closely associated with high levels of cardiovascular disease morbidity and mortality in patients with CKD, particularly in patients with end-stage renal disease. In addition, the expressions of α-kl mRNA and α-Kl protein were severely reduced in these patients. These results suggest the involvement of α-Kl and FGF23 in the pathogeneses of not only aging-associated syndromes but also the complications of CKD. Here, the unveiling of the molecular functions of α-Klotho and FGF23 has recently given new insight into the field of mineral homeostasis and the pathogeneses of aging-associated syndromes and the complications of CKD.