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href=\"https://medlineplus.gov/genetics/condition/bjornstad-syndrome\">Björnstad syndrome</a></li></ul>","identityNote":"MONDO cross-references identify native condition entries. Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>Björnstad syndrome is caused by mutations in the BCS1L gene. The protein produced from this gene is found in cell structures called mitochondria, which convert the energy from food into a form that cells can use. In mitochondria, the BCS1L protein plays a role in oxidative phosphorylation, which is a multistep process through which cells derive much of their energy. The BCS1L protein is critical for the formation of a group of proteins known as complex III, which is one of several protein complexes involved in this process. As a byproduct of its action in oxidative phosphorylation, complex III produces reactive oxygen species, which are harmful molecules that can damage DNA and tissues.</p><p>BCS1L gene mutations involved in Björnstad syndrome alter the BCS1L protein and impair its ability to aid in complex III formation. The resulting decrease in complex III activity reduces oxidative phosphorylation. For unknown reasons, overall production of reactive oxygen species is increased, although production by complex III is reduced. Researchers believe that tissues in the inner ears and hair follicles are particularly sensitive to reactive oxygen species and are damaged by the abnormal amount of these molecules, leading to the characteristic features of Björnstad syndrome.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/bjornstad-syndrome/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Björnstad syndrome?"},{"bodyHtml":"<p>People with Björnstad syndrome also have hearing problems that become evident in early childhood. The hearing loss, which is caused by changes in the inner ear (sensorineural deafness), can range from mild to severe. Mildly affected individuals may be unable to hear sounds at certain frequencies, while severely affected individuals may not be able to hear at all.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/bjornstad-syndrome\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"bjornstad-syndrome","sourceDocuments":[{"name":"Björnstad syndrome","reviewed":"2014-03","sections":{"causes":{"fetchedAt":"2026-10-20T20:19:52.863377+00:00","label":"Causes","pageSha256":"eb305dd058eb1a4973695b9667469e2bed1b6aa01124ce6407e047f9143c7c18","sourceUrl":"https://medlineplus.gov/genetics/condition/bjornstad-syndrome/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/bjornstad-syndrome"}],"sourceNote":"MedlinePlus descriptions and Causes; original HPO annotations.","takeawayHtml":"Cause explanations are from MedlinePlus Genetics. Feature values below retain their original sources and patient groups.","title":"Björnstad syndrome"},"schemaVersion":1}