FMR1 and fragile X syndrome: Current diagnostic testing for FXS usually involves a stepwise process of first determining CGG repeat size, usually by AmplideX PCR or another PCR based method, followed by assaying FMR1 promoter methylation status using AmplideX mPCR with or without Southern blot based analysis targeting the FMR1 CpG island (with the analytical sensitivity at 5–20%).