Mental retardation, X-linked, syndromic, Snyder-Robinson type (MRXSSR; MIM 309583) also known as Snyder-Robinson mental retardation syndrome (SRS) is an X-linked recessive disorder. It is characterized mainly by neurological, skeletal and distinctive facial features. The onset of developmental delay and intellectual disability is in early childhood. SRS is typically associated with hypotonia and unsteady gait. Additional findings include low muscle mass, kyphoscoliosis, osteoporosis, multiple fractures, pectus deformities, facial asymmetry, prominent lower lip, and cleft palate. SRS is caused by mutations in the SMS gene, which codes for spermine synthase.