Exon 27 was replaced with a modified exon 27 in which the four conserved TTAG base pairs were deleted (corresponding to the c.2708_2711delTTAG mutation described in patients with dominant optic atrophy) and a loxP-flanked neomycin cassette was inserted between exon 25 and 26. The neomycin cassette was removed via Cre-mediated recombination. This deletion results in a frame shift, leading to the loss of the last 58 amino acids of all isoforms. (J:237963)