- Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia. Longley, M.J., Clark, S., Yu Wai Man, C., Hudson, G., Durham, S.E., Taylor, R.W., Nightingale, S., Turnbull, D.M., Copeland, W.C., Chinnery, P.F. Am. J. Hum. Genet. (2006)