Nuclear transfer (NT) has been proposed as a strategy to overcome transmission of debilitating, maternally inherited mitochondrial DNA (mtDNA) diseases. This technique consists of transferring the nuclear genome from the egg or zygote of an affected woman to an enucleated egg or zygote from an unaffected donor. To date, the main problem with the procedure is that the replacement of mutant mtDNA is not complete: some of the mutated mtDNA surrounding the nucleus, termed carryover, will inevitably persist. Concerns are growing about these small amounts of mutant mtDNA transferred during NT because they may undergo preferential replication and outcompete healthy mitochondria from the donor egg, leading to reversion to the disease-causing genotype in the child. According to recent observations, this phenomenon appears to happen after embryo transfer, coinciding with the resumption of mtDNA replication. However, the exact timing and mechanism by which this occurs are yet to be elucidated. To address these questions and gain a better understanding on mtDNA genetic drift, I will investigate mtDNA segregation patterns at the pre- and post-implantation stages in human embryos. In addition, I will explore the use of mito-transcription activator-like effector nuclease (mito-TALENs), a novel gene editing approach, to eliminate carryover mtDNA after NT in a mouse model, thereby preventing the risk of reversion to the mutant haplotype.