Telomerase reverse transcriptase (TERT) is a 127 kDa member of the reverse transcriptase family of ribonucleoprotein enzymes that are essential for replication of chromosome termini. Human TERT is 1132 amino acids (aa) in length and contains one reverse transcriptase domain (aa 605-935). Splicing variants produce three isoforms for human TERT. Isoform 1 is the long form. Isoform 2 has a 44 aa substitution for aa 764-807 in isoform 1, and a deletion of aa 808-1132. Isoform 3 has a deletion of aa 885-947. Human TERT shares 62% aa sequence identity with mouse and rat TERT. Defects in TERT are associated with a susceptibility to aplastic anemia, are a cause of dyskeratosis congenital autosomal dominant, and increase susceptibility to idiopathic pulmonary fibrosis.