A significant increase in the phosphorylation of Ser21-GSKα and Ser9-GSKβ was reported in the Fmr1 knock-out mouse, and MPEP treatment did not alter the expression of GSK3, indicating that both lithium and MPEP are both promising candidates for ameliorating FXS symptoms [132,133,134]. Here, FMR1 is linked to fragile X syndrome.