NDUFA9 and neurodegenerative disease: The mitochondrial dysfunction in the neurodegenerative diseases cluster (Cluster 4) comprises 95 genes, including several key mitochondrial protein genes acting as pivotal hub genes, such as Sdhb, Ndufa4, Ndufb4c, Ndufb4b, Ndufv3, Ndufa9, Ndufc1, Ndufs5, Ndufb4, Ndufa12, Ndufa11, and Ndufb1.