AIM:To investigate the prevalence of fragile X syndrome(FXS)in intellectually disabled male and female Indonesians.METHODS:This research is an extension of a previously reported study on the identification of chromosomal aberrations in a large cohort of 527 Indonesians with intellectual disability(ID).In this previous study,87 patients had a chromosomal abnormality,five of whom expressed fragile sites on Xq27.3.Since FXS cannot always be identified by cytogenetic analysis,molecular testing of the fragile X mental retardation 1 CGG repeat was performed in 440 samples.The testing was also conducted in the five previously identified samples to confirm the abnormality.In total,a molecular study was conducted in 445 samples(162 females and 283 males).RESULTS:In the cohort of Indonesian ID population,the prevalence of FXS is 9/527(1.7%).The prevalence in males and females is 1.5%(5/329)and 2%(4/198),respectively.Segregation analysis in the families and X-inactivation studies were performed.We performed the first comprehensive genetic survey of a representative sample of male and female ID individuals from institutions and special schools in Indonesia.Our findings show that a comprehensive study of FXS can be performed in a developing country like Indonesia where diagnostic facilities are limited.CONCLUSION:The prevalence of FXS is equal in females and males in our study,which suggests that the prevalence of FXS in females could be underestimated.