Deoxycytidine, also known as dC, belongs to the class of organic compounds known as pyrimidine 2'-deoxyribonucleosides. Pyrimidine 2'-deoxyribonucleosides are compounds consisting of a pyrimidine linked to a ribose which lacks a hydroxyl group at position 2. Deoxycytidine is also classified as a deoxyribonucleoside, a component of deoxyribonucleic acid (DNA). Deoxycytidine is similar to the ribonucleoside cytidine, but with one hydroxyl group removed from the 2' position. Deoxycytidine exists in all living species, ranging from bacteria to plants to humans. Degradation of DNA through apoptosis or cell death produces deoxycytidine. Within humans, deoxycytidine participates in a number of enzymatic reactions. In particular, deoxycytidine can be biosynthesized from dCMP through the action of the enzyme cytosolic purine 5'-nucleotidase. In addition, deoxycytidine can be converted into dCMP; which is mediated by the enzyme uridine-cytidine kinase-like 1. Deoxycytidine can be phosphorylated at the C-5 position by the enzyme deoxycytidine kinase to produce deoxycytidine monophosphate (dCMP), and to a lesser extent, deoxycytidine diphosphate (dCDP), and deoxycytidine triphosphate (dCTP). Deoxycytidine can also be phosphorylated by thymidine kinase 2 (TK2). Deoxycytidine can potentially be used for the treatment of the metabolic disorder known as thymidine kinase 2 deficiency (TK2 deficiency). TK2 deficiency has three disease subtypes: i) infantile-onset myopathy with rapid progression to early death ii) childhood-onset myopathy, which resembles spinal muscular atrophy (SMA) type III, begins between ages 1 and 12 years with progression to loss of ambulation within few years and iii) late-onset myopathy starting at age 12 year or later with moderate to severe myopathy manifesting as either isolated chronic progressive external ophthalmoplegia (CPEO) or a generalized myopathy with CPEO plus facial and limb weakness, gradual progression, and, in some cases, respiratory failure and loss of ability to walk in adulthood (PMID: 28318037). In mouse models of TK2, dC was shown to delay disease onset, prolong life span and restore mtDNA copy number as well as respiratory chain enzyme activities (PMID: 28318037).