Mitochondria, among other functions, generate energy in the form of ATP. The chondrial genome, located within each mitochondrion, encodes some of the polypeptides associated with the electron transfer chain (ETC) and ATP production. Transcription and replication of mitochondrial DNA (mtDNA) is dependent upon the import of transcription and replication factors encoded by the nucleus. Certain point mutations and large-scale deletions to mtDNA can be either severely debilitating or lethal. The transmission and inheritance of mtDNA [not readable: see to offspring is strictly regulated and specific to each species. In many mammalian systems, paternal mtDNA is eliminated very early during embryonic development. However, it is possible that the paternal molecule could be extruded to those cells destined to become trophoblasts and may act as a regulator of embryonic cell fate. Furthermore, the increasing use of more sophisticated assisted reproductive techniques has led to the incorporation of extraneous mtDNA in both the reconstructed oocyte and embryo with transmission to the offspring at varying degrees.