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As discussed above, the amount of mtDNA copy number present in the oocyte at the time of fertilisation is important to subsequent developmental outcome. There are, perhaps, three key reasons for this. Firstly, establishment of the appropriate number of mtDNA copies in the metaphase II oocyte aids the process of developmental competence, as there appears to be a prerequisite number of mtDNA copies required for developmental competence to be established (41, 72). Secondly, a refined number of mtDNA copies are required for successful fertilisation outcome, as demonstrated in a number of mammalian species including human (41, 73-76). Thirdly, the establishment of sufficient mtDNA copy number is an investment in subsequent developmental outcome (72). The failure to possess sufficient copies of mtDNA at fertilisation, i.e. mtDNA deficiency, exists for a number of women, and appears to be more prevalent in women of older reproductive age (77). The consequence is that their oocytes either fail to fertilise or they arrest during preimplantation development (73, 75, 78).