Understanding mitochondrial diseases, which are estimated to affect about one in 4,000 people, remains a significant challenge because of the diversity of human disorders at every level that result when the mitochondria of the cell cannot produce the energy the body needs to support growth. In part one, a number of therapies were described that have advanced through to the clinic that target Friedrich's ataxia and other rare mitochondrial diseases. (See BioWorld Insight, Sept. 18, 2017.)