HTT and Huntington disease: In our MFA, we examined the relationship between four groups: mitochondrial pathways (variables: protein expression data by proteomics of 39 proteins); mitochondrial mass indicators (variables: mtDNA copy number and citrate synthase activity); mitochondrial activity (variables: mitochondrial respiratory chain complex activities); and HD biology (huntingtin CAG repeat lengths, mutant huntingtin protein levels) (Fig. 1A and Supplementary Table 6).