The function of ENSG00000107815 (TWNK, twinkle mtDNA helicase) is as follows. [Isoform 1]: Mitochondrial helicase involved in mtDNA replication and repair (PubMed:12975372, PubMed:15167897, PubMed:17324440, PubMed:18039713, PubMed:18971204, PubMed:25824949, PubMed:26887820, PubMed:27226550). Might have a role in mtDNA repair (PubMed:27226550). Has DNA strand separation activity needed to form a processive replication fork for leading strand synthesis which is catalyzed by the formation of a replisome complex with POLG and mtSDB (PubMed:12975372, PubMed:15167897, PubMed:18039713, PubMed:22383523, PubMed:26887820, PubMed:27226550). Preferentially unwinds DNA substrates with pre-existing 5'-and 3'- single-stranded tails but is also active on a 5'- flap substrate (PubMed:12975372, PubMed:15167897, PubMed:18039713, PubMed:22383523, PubMed:26887820, PubMed:27226550). Can dissociate the invading strand of immobile or mobile D-loop DNA structures irrespective of the single strand polarity of the third strand (PubMed:27226550). In addition to its DNA strand separation activity, also has DNA strand annealing, DNA strand-exchange and DNA branch migration activities (PubMed:22383523, PubMed:26887820, PubMed:27226550). {ECO:0000269|PubMed:12975372, ECO:0000269|PubMed:15167897, ECO:0000269|PubMed:17324440, ECO:0000269|PubMed:18039713, ECO:0000269|PubMed:18971204, ECO:0000269|PubMed:22383523, ECO:0000269|PubMed:25824949, ECO:0000269|PubMed:26887820, ECO:0000269|PubMed:27226550}. [Isoform 2]: Lack DNA unwinding and ATP hydrolysis activities (PubMed:18039713). Does not bind single-stranded or double-stranded DNA (PubMed:18039713). .