Klotho is an enzyme that in humans is encoded by the KL gene. The three subfamilies of klotho are α-klotho, β-klotho, and γ-klotho. α-klotho activates FGF23, and β-klotho activates FGF19 and FGF21. When the subfamily is not specified, the word "klotho" typically refers to the α-klotho subfamily, because α-klotho was discovered before the other members.

α-klotho is highly expressed in the brain, liver and kidney. β-klotho is predominantly expressed in the liver. γ-klotho is expressed in the skin.

Klotho can exist in a membrane-bound form or a (hormonal) soluble, circulating form. Proteases can convert the membrane-bound form into the circulating form.

The KL gene encodes a type-I single-pass transmembrane protein that is related to β-glucuronidases. Reduced production of this protein has been observed in patients with chronic kidney failure (CKF), and this may be one of the factors underlying degenerative processes (e.g., arteriosclerosis, osteoporosis, and skin atrophy) seen in CKF. Mutations within the family have been associated with ageing, bone loss and alcohol consumption. Transgenic mice that overexpress klotho live longer than wild-type mice.

The α-klotho gene is located on chromosome 13, and is translated into a single-pass integral membrane protein.

The β-Klotho gene is located on chromosome 4. The protein shares homology (43.1% identity and 60.1% similarity) with α-klotho. The β-Klotho gene and β-Klotho protein should not be confused with the alpha-cut and beta-cut of alpha-klotho, which releases KL1+KL2 and KL2 domain of α-klotho into the extracellular matrix and bloodstream, respectively.

There are two main isoforms.

One isoform is the full-length Klotho mRNA, that includes signal peptide, KL1 domain, linker region, KL2 domain, transmembrane region and intracellular domain. This gets produced into a full protein that is stuck in the membrane and subsequently is cleaved into soluble klotho proteins that are found in circulation in the bloodstream.