Task: Question Answering
Input (English): Context: Christianson syndrome is an X-linked mental retardation syndrome characterized by microcephaly, impaired ocular movement, severe global developmental delay, hypotonia which progresses to spasticity, and early onset seizures of variable types. Gilfillan et al.2008] reported mutations in SLC9A6, the gene encoding the sodium/hydrogen exchanger NHE6, in the family first reported and in three others. They also noted the clinical similarities to Angelman syndrome and found cerebellar atrophy on MRI and elevated glutamate/glutamine in the basal ganglia on MRS. Here we report on nonsense mutations in two additional families. The natural history is detailed in childhood and adult life, the similarities to Angelman syndrome confirmed, and the MRI/MRS findings documented in three affected boys. 
 Question: Mutation of which gene is implicated in the Christianson syndrome?
Output (English): slc9a6