Of the ten nuclear‐encoded structural subunits of CIII, only variants in UQCRC2, UQCRB, UQCRQ and CYC1 have been reported in mitochondrial disease cases associated with hypoglycaemia, lactic acidosis, ketosis and hyperammonaemia (Haut et al, 2003; Barel et al, 2008; Gaignard et al, 2013; Miyake et al, 2013). Here, CYC1 is linked to mitochondrial disease.