CTBP1 and mitochondrial disease: The identification of a pathogenic, heterozygous, de novo variant in CTBP1 by whole exome sequencing in an individual with a neurodevelopmental phenotype and muscle biopsy suggestive of mitochondrial disease is a prime example of the importance of closely analysing de novo variants for pathogenicity and additionally, considering secondary mitochondrial dysfunction in genes not directly involved in OXPHOS.