Hydroxysteroid (17) dehydrogenase 10 (HSD10) deficiency (formerly called 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency) is a very rare disorder with very few patients described caused by mutations in the HSD17B10 gene located on the X chromosome. Most patients develop severe neurological abnormalities, such as the gradual loss of mental and motor skills that progresses to profound developmental regression, choreoathetosis, near blindness and epilepsy. Cardiomyopathy has also been described. Males are more severely affected than females, with females presenting a variety of symptoms: from borderline learning difficulties to psychomotor and speech delay. Mutation in the HSD17B10 gene has also been associated with a mild X-linked mental retardation syndrome with choreoathetosis and abnormal behavior, involving aggressiveness, hallucinations, self mutilation and speech impairment called MRXS10. (MIM# 300220).