After we started this project, it was reported that POLG is one the of causative genes for hCPEO (Van Goethem et al., 2001). In one of these pedigrees of POLG mutations, affected family members also had depression. This enhanced the construct validity of this model.The transgenic mice showed age-dependent accumulation of mtDNA deletions in the brain (Kasahara et al., 2006). The partially deleted mtDNA molecules detected by Southern blot analysis were about 2 kb, which suggested that a large portion (14 kb) is deleted from the 16-kb mtDNA molecule. The size of deletion is larger than the typical deletions found in patients with CPEO, 5–7 kb. The mutation of Polg, D181A, corresponding to D198A in humans, has not been reported in patients with CPEO. This mutation might cause more severe disturbances than those reported in the patients.