The Drosophila mutant technical knockout (tko), affecting the mitochondrial protein synthetic apparatus, exhibits respiratory chain deficiency and a phenotype resembling various features of mitochondrial disease in humans (paralytic seizures, deafness, developmental retardation). We are using this mutant to analyse the cellular and genomic targets of mitochondrial dysfunction, and to identify ways in which the phenotype can be alleviated. Transgenic expression of wild-type tko in different patterns in the mutant background reveals critical times and cell-types for production of components of the mitochondrial disease-like phenotype. Mitochondrial bioenergy deficit during the period of maximal growth, as well as in specific parts of the nervous system, appears to be most deleterious. Inbreeding of tko mutant lines results in a systematic improvement in all phenotypic parameters tested. The resulting sub-lines can be used for genetic mapping and transcriptomic analysis, revealing clues as to the genes and pathways that can modify mitochondrial disease-like phenotypes in a model metazoan.