Classical Rett syndrome in sisters: variability of clinical expression.

Familial cases of Rett syndrome (RS) are rare. No significant differences have been reported in the clinical courses of concordant monozygotic twins with RS. We present the variability of clinical expression in two Japanese sisters with classic RS. The younger sister, currently 6 years and 6 months old, never stood or walked alone, showed severe spasticity, growth retardation, and microcephaly and developed sleep-wake rhythm disturbance from age 4 years and seizures from age 5 years. The elder, currently 7 years and 9 months old, walked alone and had mild spasticity, no growth retardation, normal sleep-wakefulness rhythm and no seizures. RS is most likely to be transmitted as an X-linked dominant, male-lethal (XDML) disorder, although this is still contested. If RS is an XDML disorder, lyonization may account for variability of expression in the sisters.