FMR1 and fragile X syndrome: Consistent with findings in brain tissue from Fragile X syndrome subjects (Irwin et al., 2001), studies on L5 cortical neurons found that Fmr1 knockout mice display increased spine densities with immature, abnormally elongated, spine morphologies even at adult ages (Comery et al., 1997; Nimchinsky et al., 2001; Galvez and Greenough, 2005).