The mitochondria has a genome (mtDNA). Not my area but I think while we initially thought it was a fossil of the mito's prokaryotic history and we'd subsumed all the good stuff, there are genes there that may be of interest for exploring ageing and metabolic disorders. It's also quite interesting genetically, as only passed down the maternal line, so can be traced back to a single "mitochondrial eve" (MRCA). This brings up some neat ideas in selfish gene theory wrt battle of the sexes etc.