FMR1 and Intellectual disability: An important feature of this dataset is that the regression line for IQ passes the lower end of the normal IQ range (IQ = 85) at approximately 2 SD below the normal mean FMRP level and passes the cut-off for intellectual disability (IQ = 70) at approximately 2.5 SD below the normal mean level of FMRP (Fig 2 and Table 2).