Abnormalities of the nervous system, including neurodevelopmental abnormality (HP:0012759), ataxia (HP:0001251), seizure (HP:0001250), peripheral neuropathy (HP:0009830) etc., were present in 44% of the families reported, and the top five contributing genes were POLG, SPG7, MFN2, FXN and C19ORF12 (Fig. 8). This evidence concerns the gene POLG and cerebellar ataxia.