LHON has been suggested to involve both mitochondrial and X-chromosome-linked loci. By extending two-locus mitochondrial and nuclear gene analytic methods, we recently proposed that a proportion of affected females are likely heterozygous at the X-linked locus and affected due to unfortunate X-chromosome inactivation. Assuming that the optic tissue is the primary site of action of the mutant gene(s), we further propose here that there should be no fewer than six embryonic precursor cells for the involved optic tissue at the stage in early development when X-chromosome inactivation occurs. We also estimate that the disease threshold (i.e. proportion of cells with abnormal X-chromosome active in the responsible tissue at the time of X-chromosome inactivation) for a heterozygous female is in the range of 0.60 to 0.83.