In the screening of RRM2B variants in 50 mitochondrial disease patients without causative variants in POLG1 and C10orf2, one Kearns-Sayre syndrome (MIM 530000) patient who carried two different novel missense variants and one PEO patient who carried an in-frame deletion were identified [25]. The gene discussed is RRM2B; the disease is Kearns-Sayre syndrome.