Fragile X Syndrome is caused by transcriptional silencing or loss-of-function mutations in the FMRP gene, which encodes for the Fragile X mental retardation protein (FMRP) that is required for the activity-dependent synapse elimination by PCDH10, triggered following the activation of the transcription factor myocyte enhancer factor 2 (mef2) [116]. The gene discussed is PCDH10; the disease is fragile X syndrome.