Beck-Fahrner syndrome (BEFAHRS) is a rare genetic disorder caused by a variant (also known as a mutation) in the TET3 (Tet Methylcytosine Dioxygenase 3) gene. The TET3 gene is a protein-coding gene and plays a critical role in the developing brain and in neuronal functioning.

In the case of Beck-Fahrner syndrome, a mutation in the TET3 gene causes the TET3 protein to malfunction or to not to be produced sufficiently, ultimately disrupting normal development and causing health problems.

Beck-Fahrner syndrome was first identified in 2020 and is characterized by global developmental delay and/or intellectually disability with mild to severe intellectual impairment. Affected individuals often have behavioral manifestations, such as autistic features, anxiety, or attention deficit-hyperactivity disorder (ADHD). Most patients have hypotonia (low muscle tone) and distinctive facial features. Some may have seizures and other movement disorders, joint hypermobility, poor feeding, and growth abnormalities, including overgrowth or poor growth.

As of 2021, there are only around 50 individuals in the world that have been genetically diagnosed with Beck-Fahrner syndrome. It is highly likely that many individuals with Beck-Fahrner syndrome remain undiagnosed or are misdiagnosed with other disorders. There is no cure for Beck-Fahrner syndrome. However, a diagnosis allows for appropriate monitoring and treatment of associated symptoms.