<P> Data on intellectual development in FXS are limited . However, there is some evidence that standardized IQ decreases over time in the majority of cases, apparently as a result of slowed intellectual development . A longitudinal study looking at pairs of siblings where one child was affected and the other was not found that affected children had an intellectual learning rate which was 55% slower than unaffected children . </P> <P> When both autism and FXS are present, a greater language deficit and lower IQ is observed as compared to children with only FXS . </P> <P> Fragile X syndrome co-occurs with autism in many cases and is a suspected genetic cause of the autism in these cases . This finding has resulted in screening for FMR1 mutation to be considered mandatory in children diagnosed with autism . Of those with fragile X syndrome, prevalence of concurrent autism spectrum disorder (ASD) has been estimated to be between 15 and 60%, with the variation due to differences in diagnostic methods and the high frequency of autistic features in individuals with fragile X syndrome not meeting the DSM criteria for an ASD . </P> <P> Although individuals with FXS have difficulties in forming friendships, those with FXS and ASD characteristically also have difficulties with reciprocal conversation with their peers . Social withdrawal behaviors, including avoidance and indifference, appear to be the best predictors of ASD in FXS, with avoidance appearing to be correlated more with social anxiety while indifference was more strongly correlated to severe ASD . When both autism and FXS are present, a greater language deficit and lower IQ is observed as compared to children with only FXS . </P>

What type of mutation is fragile x syndrome