FMR1 and fragile X syndrome: In addition, to ensure that the differences we observe in FXS patient lines are due to the silencing of the FMR1 gene and the absence of FMRP and not differences in the different genetic backgrounds or ‘environmental’ influences on the iPSC-derived neurons, we also describe the properties of an isogenic pair of lines [24], one of which has had the FMR1 gene genetically deleted (FMR1−/y) to allow direct comparison with an otherwise genetically identical line (FMR1+/y).