Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes - abbreted to MELAS - is one of the family of mitochondrial cytopathies, which also include MERRF, and Leber's hereditary optic neuropathy. It was first characterized under this name in 1984. A feature of these diseases is that they are caused by defects in the mitochondrial genome which is inherited purely from the female parent. However, it is impoant to know that some of the proteins essential to normal mitochondrial function are produced by the nuclear genome, and are subsequently transpoed to the mitochondria for use. As such, mutations in these proteins can result in mitochondrial disorders, but can be inherited from both male and female parent in the typical fashion. The disease can manifest in both sexes.This condition is inherited in a mitochondrial pattern, which is also known as maternal inheritance and heteroplasmy. This pattern of inheritance applies to genes contained in mitochondrial DNA. Because egg cells, but not sperm cells, contribute mitochondria to the developing embryo, only females pass mitochondrial conditions to their children. Mitochondrial disorders can appear in every generation of a family and can affect both males and females, but fathers do not pass mitochondrial traits to their children. In most cases, people with MELAS inherit an altered mitochondrial gene from their mother. Less commonly, the disorder results from a new mutation in a mitochondrial gene and occurs in people with no family history of MELAS.