Consequently, patients with the subtype of ASD with overlapping FXS, as well as Fmr1 knock-out (KO) mice, with no or a very low level of FMRP in the brain tissues, such as the hippocampus and cortex, exhibited compromised dendritic and axonal arborization, immature spine and synapses, learning/memory difficulty, hyperactivity, and cognitive impairment [31]. Here, FMR1 is linked to fragile X syndrome.