OPA1 and optic atrophy: The biallelic OPA1 variants found in two of our patients combine a classic OPA1 haploinsufficiency mutation (i.e. a frameshift change in P1 and a missense change, acting as a null allele in yeast, in P2), which can determine dominant isolated optic atrophy with reduced penetrance, and the missense mutation p.Ile437Met, considered hypomorphic or with very low potential pathogenicity since apparently it was not able to lead to clinical symptoms in heterozygous or even homozygous state [23].