The present status of efforts in the USA to understand the genetic basis of the Rett syndrome (RS) is reviewed. Analyses of monozygotic and dizygotic twin data provide compelling support for a genetic mechanism. Similarly, support can be derived from familial cases following maternal lines. Special attention is directed to the X chromosome. Nonrandom X inactivation in the mother of half sisters and the identification of RS girls with translocations involving the X chromosome and separate autosomes have sharpened the focus molecular studies.