FMR1 and fragile X syndrome: To test this hypothesis, we used the CRISPR/Cas9 system to generate MSH2 knockout (KO) cell lines from FXS ESCs with 400 CGG repeats in the FMR1 gene and from induced pluripotent stem cells (iPSCs) derived from FRDA fibroblasts with ~ 800 GAA repeats in the FXN gene.