FMR1 and fragile X syndrome: In particular, the absence of RA-RARα-regulated homeostatic plasticity mechanisms have been established in both Fmr1 knockout mice (a mouse model for fragile X syndrome [FXS]) and human neurons differentiated from FXS patient-derived induced pluripotent stem cells (Soden and Chen, 2010; Zhang et al., 2013; Zhong et al., 2018).