At present, therefore, it is possible to diagnose chromosome deletion carriers using ESD quantitation and also to follow the inheritance of the predisposition to Rb in a proportion of families. We have recently shown that both quantitative and qualitative analysis of the ESD enzyme can be carried out on samples derived from the chorionic villus (CV)12 as well as cultured CV samples. The CV tissue is produced by the fetus and destined to become the placenta. Most importantly, CV samples can be obtained in the first trimester of pregnancy, after only 6-8 weeks. Thus, a diagnosis can be made early in pregnancy compared with the 18-20 weeks which must elapse before fetal blood samples can be taken. The same CV samples could be used to prepare DNA which can be used for carrier detection using DNA probes as described above. We are, therefore, moving rapidly towards the time when it will be possible to offer prenatal diagnosis to all familial cases of Rb. A remaining challenge will be to determine the nature of the genetic changes which occur within the Rb predisposition gene leading to abnormal development of the retina and consequently tumour development. It may then be possible to determine whether isolated cases of Rb are truly sporadic or the beginning of a new hereditary line.
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