POLG and Friedreich ataxia: The finding of a recessive mutation in a heterozygous state in controls does not per se refute a possible pathogenic role, in particular if the variant is common (as e.g. demonstrated by the expanded allele of FXN [Friedreich’s ataxia gene], which is observed with a heterozygous carrier rate of 1/60 to 1/90 in controls [24], or by the two frequent, clearly pathogenic c.1399 G>A [p.Ala467Thr] and c.2243 G>C [p.Trp748Ser] POLG mutations, which are observed with a heterozygous carrier rate of up to 1% in several European populations [25]).