The potential involvement of klotho in the calcium–phosphorus–vitamin D homeostatic system. PTH, parathyroid hormone; TRPV5, TRPV6, transient receptor potential cation channel, subfamily V, members 5 and 6 respectively; 1αOHase, 25-hydroxyvitamin D-1α-hydroxylase; 1,25(OH)2D, 1,25-dihydroxyvitamin D; NTP2a, NTP2b, Na–phosphate cotransporters; Pi, inorganic phosphorus; FGF, fibroblast growth factor; FGFR, FGF receptor; +, increased; −, decreased.