Another fundamental difference is that Fmr1 KO mice never produce FMRP from birth, although the Fmr1 promoter remains intact, leading to residual transcription of abnormal Fmr1 RNA at around 27% of wildtype levels [12,13], whereas FXS individuals have active FMR1 expression until at least the 10th week of gestation [93,94]. Here, FMR1 is linked to fragile X syndrome.