The problem sits on an X Chromosome  of which girls have two.  Boys have only one X chromosome (and one Y chromosome) so they lack a back-up copy that could compensate for the defective one.

If a boy carries a similar MECP2 mutation (to those found in females with Rett syndrome) on his single X chromosome, with no other ‘normal’ X chromosome to compensate, then he will usually be affected much more severely even than a female.

Sometimes a boy will be born with a MECP2 alteration, usually found in those with classic RTT, but where it often has a less severe effect than some of the other gene alterations.

For the clinical features of Rett syndrome to develop in a boy, the mutation should usually be present and expressed in just some of his cells – as happens in a female.

The incidence of Rett syndrome in boys is believed to be around 1:40,000.