FMR1 and fragile X syndrome: Regarding the genetic factors, various genetic syndromes can result in intellectual impairment, such as DS, caused by chromosomal variation (another chromosome 21) (Chen et al., 2014), Williams syndrome caused by deletion of the region near the long-arm chromosome 7 (7q11.23) (Neuman and Henske, 2011), FXS caused by FMR1 gene defect and its protein product deletion (Pacey et al., 2015), RS caused by the MECP2 gene mutation on the X chromosome (Andoh-Noda et al., 2015), and tuberous sclerosis (TS) caused by TSC1 and TSC2 gene mutations (Neuman and Henske, 2011).