mitochondrial complex 2 deficiency, nuclear type 4 (Monarch Disease Ontology term MONDO_0030974) is an autosomal recessive disorder due to pathogenic variants in the SDHB gene, resulting in Mitochondrial complex II deficiency and a variety of clinical manifestations, including neurological and muscular symptoms. Also known as: MC2DN4, SDHB-related Nuclear type mitocondrial complex II deficiency, mitochondrial complex II deficiency, nuclear type 4.