The reason why it's just girls is because the gene is carried on the X chromosome- therefore, girls can carry a functional copy on the other X chromosome, but most guys (not counting Klinefelter types) don't have that option. Males who inherit the bad X die in infancy. I'm guessing that people figured out their deceased baby boys had the RTT problem via genetic testing. See 'Gender and Rett Syndrome' here- http://en.wikipedia.org/wiki/Rett_syndrome .
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The reason why it's just girls is because the gene is carried on the X chromosome- therefore, girls can carry a functional copy on the other X chromosome, but most guys (not counting Klinefelter types) don't have that option. Males who inherit the bad X die in infancy. I'm guessing that people figured out their deceased baby boys had the RTT problem via genetic testing. See 'Gender and Rett Syndrome' here- http://en.wikipedia.org/wiki/Rett_syndrome .