The synaptophysin gene (SYP) encodes an integral membrane protein that is associated with small synaptic vesicles in the brain and endocrine cells. The SYP gene is located at Xp11.23-p11.2 and contains 7 exons. Mutations of various types in the SYP gene are associated with non-syndromic X-linked mental retardation (Tarpey PS et al. Nat Genet. 2009;41:535-543).