Thus, the autozygome provides a unique opportunity to unmask the phenotypic expression of variants that had only been reported as compound heterozygous.39 For example, we show here that one PARS2 allele that was reported to cause Alpers syndrome when compound heterozygous only causes nonspecific global developmental delay and brain atrophy when homozygous but with no evidence of lactic acidosis or liver involvement when homozygous. The gene discussed is PARS2; the disease is lactic acidosis.