When the mother is a carrier of an X-linked disease, there is a 50% chance for her son to be affected. Prenatal sex determination by chromosome analysis may help these parents to limit their families to unaffected daughters (who will be carriers in 50% of the cases). The limitations of this approach are ob­vious, but it is the only possibility for these parents to have their own child­ren, without being forced to accept a high risk for a severely affected child. This approach applies to those X-linked disorders where the basic defect is un­known, (e.g. Duchenne's muscular dystrophy, X-linked mental retardation, etc.) or is not yet detectable in amniotic fluid or cultured amniotic fluid cells (e.g. hemophilia). In a few X-linked diseases the biochemical defect is expressed in cultured amniotic fluid cells and this allows prenatal identifica­tion of an affected male fetus (Fabry's disease; Hunter's disease; the Lesch-Nyhan syndrome; for references: see Burton et al., 1974).