Xq28

Xq28 is a cytogenetic band on the X chromosome, located at the distal end of the long arm (the q arm). In chromosome band nomenclature, Xq28 denotes the most distal subdivision of Xq and is used in karyotyping and molecular cytogenetics. The region is gene-rich and has been a focus in studies of X-linked disorders and neurodevelopmental conditions.

The region contains many genes, among which MECP2, L1CAM, and GDI1 are notable. MECP2 mutations in Xq28 are a common cause of Rett syndrome and related neurodevelopmental disorders. L1CAM, located in Xq28, encodes a neural cell adhesion molecule; pathogenic variants cause X-linked hydrocephalus and related disorders (L1 syndrome). GDI1, also in Xq28, encodes a GDP dissociation inhibitor; mutations are associated with X-linked intellectual disability and seizures in males.

Because of its gene content, Xq28 is a region frequently analyzed in genetic testing for X-linked neurodevelopmental conditions. Structural rearrangements, microdeletions, and duplications in Xq28 have been reported in patients with developmental delay, autism spectrum features, and intellectual disability. The region is assessed by karyotype analysis, fluorescence in situ hybridization (FISH), and array comparative genomic hybridization (array CGH) or sequencing when X-linked disorders are suspected.