how can lhon be homoplasmic for a mutation if having all mutant mitochondria is lethal

how can lhon be homoplasmic for a mutation if having all mutant mitochondria is lethal

I learned in genetics that being homoplasmic for all mutant mitochondria is lethal, but later I read that Leber’s Hereditary Optic Neuropathy is homoplasmic for a mutation. How can this be? 

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