In the human genome, miR-504 is an intronic miRNA, hosted by the fibroblast growth factor 13 gene (FGF13) on chromosome Xq26.3, which is highly expressed in the brain and was previously associated with Börjeson–Forssman–Lehmann syndrome, an X-linked mental retardation [54]. This evidence concerns the gene FGF13 and Borjeson-Forssman-Lehmann syndrome.