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Prevalence
SLC9A6-related disorder, also called Christianson syndrome, is rare. Its prevalence is uncertain but is estimated to range between 1 in 16,000 and 100,000 people.

Clinical characteristics
Male individuals with Christianson syndrome have moderate to severe developmental delay / intellectual disability, epilepsy, regression in early childhood, mutism, postnatal microcephaly, hyperkinesis, truncal ataxia, and cerebellar atrophy. Secondary symptoms include autistic behaviour, ophthalmoplegia (strabismus), gastroesophageal reflux disease, hypotonia, low weight for age group, difficulty sleeping, and hypoalgesia.

Females who carry a defective variant of SLC9A6are generally asymptomatic but in exceptional cases can exhibit moderate intellectual disability, delays in speech and language acquisition, and psychiatric illnesses, including separation anxiety disorder, difficulty forming emotional attachments to others, uncooperative behaviour and attention deficit/hyperactivity disorder (ADHD).

Inheritance
Since the SLC9A6gene is located on the X chromosome, SLC9A6-related conditions are known as X-linked conditions. As males have only one X chromosome, those possessing a defective variant of SLC9A6are invariably affected. Females who are genetic carriers of the condition will have one X chromosome with a SLC9A6variant, while the other X chromosome will contain a functional copy of the gene. Hence, female carriers may not show any signs or symptoms of the condition. When a female who is carrying a SLC9A6variant has children and the variant is passed on to her children, then a male child will have a SLC9A6-related condition.