RESUMEN
Rett syndrome is a progressive encephalopathy restricted to the female sex. In the present paper a possible genetic cause for this syndrome is discussed, based on data from the literature as well as our own. Our results are in agreement with others regarding no increase in parental age, or in spontaneous abortions rate among the mothers of affected children and with a normal sex ratio among sibs. We have found no chromosome rearrangement detectable with the methods used and no correlation between fra(X) (p22) and the Rett syndrome. We have observed an alteration in the sequence of replication in one of the two types of late-replicating X-chromosome present in normal women, and suggest that this may signify that genes which are active in the late-replicating X-chromosome are inactivated (or vice-versa) in these patients. This fact could be related to the abnormal phenotype observed in Rett syndrome patients.