Cybrids, or CYtoplasmic hyBRIDs, are eukaryotic cell lines produced by the fusion of rho-zero cells and mitochondria from another donor. Rho-zero cells are cells which have been deprived of their own mitochondrial DNA by prolonged incubation with ethidium bromide, a chemical which inhibits DNA replication. They do retain their own nuclear genome. A cybrid is then a hybrid cell which combines the nuclear genome from one source with mitochondrial genomes from other sources. As a result, in this model it is possible to dissociate the biochemical influence of the mitochondrial genome from its nuclear background. Cybrids are valuable in mitochondrial research and were used to prove mitochondrial involvement in Alzheimer's disease, Parkinson's disease, and other conditions.