Twinkle, also known as PEO1 (Progressive external ophthalmoplegia 1 protein), PEOA3, SANDO or TWINL, is a mitochondrial protein that functions as a 5’-3’ nucleotide-dependent DNA helicase. Colocalized with mtDNA (mitochondrial DNA) in mitochondrial nucleoids, Twinkle is important in the metabolism and maintenance of mtDNA, playing a crucial role in the regulation of mtDNA copy numbers. Twinkle is expressed at high levels in testis, pancreas and skeletal muscle and exists as three isoforms due to alternative splicing events. Defects in the gene encoding Twinkle are the cause of two conditions: progressive external ophthalmoplegia with mitochondrial DNA deletions autosomal dominant 3 (PEOA3) and sensory ataxic neuropathy dysarthria and ophthalmoparesis (SANDO). PEOA3 is characterized by ptosis and weak muscles, while SANDO is characterized by ophthalmoparesis, dysarthria and sensory ataxic neuropathies.