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Table 1 Summary of the location, function and expression patterns of the human genes in this study.

From: Origin and evolution of candidate mental retardation genes on the human X chromosome (MRX)

Locus symbol Gene name Human location Proposed function Expression in human and/or mouse reference
RSK2 (RPS6KA3) ribosomal S6 kinase 2 Xp22.2
Serine threonine protein kinase that phosphorylates CREB and regulates histone H3 acetyltransferase, to play a role in long term memory Expressed widely, but only at low levels in foetal and adult tissues; is concentrated in the adult brain, skeletal muscle and heart [24–26]
ARX Aristaless-related homeobox gene Xp22
Transcription factor with a role in neuronal proliferation and differentiation Restricted to embryonic brain in human, but also found in mouse testis. Widespread strong expression in the adult heart, skeletal muscle, kidney and liver, no expression in the brain. [27–30]
TSPAN7 (TM4SF2) tetraspanin 7 Xp11.4
Modulates integrin-mediated signalling, to play a role in neurite outgrowth and synapse formation Ubiquitous in foetus and adult [31]
SREB3 (GPR173) Super conserved receptor expressed in the brain Xp11.22
G- protein coupled receptor, role in MRX unknown Restricted to brain and ovary with low levels detectable in the small intestine [32, 33]
JARID1C (SMCX) Jumonji AT-rich interactive domain 1C Xp11.22
Role in chromatin remodelling by regulating transcription Widely expressed at different levels in different tissues. Strongest in adult brain and skeletal muscle. [34]
OPHN1 Oligophrenin 1 Xq12
Negatively regulates Rho-GTPase activity to affect formation and dismantling of the actin cytoskeleton and control of neurite growth Ubiquitous in foetus, restricted to central and peripheral neuronal tissues in the adult [35–37]
RSK4 (RPS6KA6) ribosomal S6 kinase 4 Xq21
Serine threonine kinase with very high homology to RSK2, suggesting a role in normal cell proliferation and neuron differentiation Widespread weak expression in adult, concentrated in the adult brain and kidney [24, 38, 39]
AGTR2 angiotensin II receptor, type 2 Xq22
Receptor protein with a role in central nervous system or cardiovascular function Highly expressed in all developing foetal tissues, but adult expression only in brain, adrenal medulla, and atretic ovary [40, 41]
ARHGEF6 rho guanine nucleotide exchange factor 6 Xq26
activates Rho-GTPase activity to affect formation and dismantling of the actin cytoskeleton and control of neurite growth Ubiquitous [35, 42, 43]
FMR1 fragile X mental retardation 1 gene Xq27.3
RNA-binding protein with roles in synaptic maturation and function, and in germ cell proliferation Widely expressed in most adult and foetal tissues, highest in brain and the testis [44–46]
FMR2 fragile site mental retardation 2 gene Xq28
Nuclear transcriptional regulator with a role in learning and memory Restricted to placenta and adult brain. [47, 48].