Reaching a full genetic diagnosis in patients with rare diseases such as the mitochondrial disorders (MDs) is important for counseling and to prepare correct follow up and future therapies. Massive gene sequencing allow extensive investigation of potential etiologies but it is important to consider uniparental disomy (UPD) as a possible explanation for the transmission whenever unusual genetic segregation results in uncertain definition of the causative variants. In this paper, we described two children with a clinical diagnosis of MD and carrying homozygous variants originated from UPD. These cases support the idea that unorthodox patterns of inheritance should be sought whenever the parental segregation raises doubts. (By Dr Filippo M M Santorelli, https://jmg.bmj.com/content/early/2021/06/15/jmedgenet-2020-107644 )