NIH

Zebra of the Week: Allan–Herndon–Dudley syndrome

February 2, 2026

Allan–Herndon–Dudley syndrome (AHDS) is a rare X‑linked disorder of brain development characterized by moderate to severe intellectual disability and significant motor impairment. The condition presents exclusively in males and disrupts neurodevelopment beginning before birth or in early infancy.

Clinically, affected individuals exhibit profound hypotonia, delayed or absent motor milestones, and limited or absent speech. As patients age, hypotonia often progresses to spasticity, joint contractures, and abnormal involuntary movements, frequently resulting in loss of independent ambulation. Despite severe neurologic impairment, many individuals demonstrate preserved social engagement and responsiveness.

AHDS is caused by pathogenic variants in SLC16A2, which encodes monocarboxylate transporter 8 (MCT8). Impaired MCT8 function disrupts thyroid hormone delivery to the developing brain, resulting in abnormal neurodevelopment. The disorder is inherited in an X‑linked recessive pattern and is estimated to affect fewer than 1,000 individuals in the U.S.

Source:

(Accessed 2026, January 30). NIH Genetic and Rare Diseases Information Center (GARD). Allan–Herndon–Dudley syndrome. https://rarediseases.info.nih.gov/diseases/5617/allan-herndon-dudley-syndrome