FMR1 and fragile X syndrome: FMRP (Fragile X mental retardation protein), the key protein missing in fragile X syndrome, is an RNA binding and transport protein that regulates the translation of many other proteins important for synaptic plasticity, including neuroligins 3 and 4 and SHANK, all of which have been previously associated with autism [12,13,139,140] Muscular dystrophy and muscle disease are also known to be among the co-morbid disorders frequently found in autism [99].