The clinical phenotypes of females with fragile X full mutations vary drastically. The authors describe a pair of discordant monozygotic female twins (dichorionic–diamniotic type) with full mutation and the degrees of their phenotypic discordance regarding physical, psychiatric and behavioural features were quantified in a series of neuropsychological test. This paper identified FMR1 expression in hair-root, which reflected FMRP expression in the brain is the most crucial factor that influences disease phenotype. The 6-fold variation in hair-root FMR1 expression may account for the disparate phenotypes in IQ, cognition and social capability between the twins.  Additionally, considering the (Di–Di type) twinning and different CGG repeat sizes, CGG expansion should occur before splitting at day 3 of gestation. (By Professor Ranhui Duan, http://jmg.bmj.com/content/early/2013/11/27/jmedgenet-2013-101978 )