We thank the Editor-in-Chief for giving us the opportunity to respond to this communication. We are, of course, pleased that Dr. Daker found our report of the Jerusalem Workshop on the Fragile X Chromosome to be "particularly interesting, and a good summary." That is what it was meant to be, and it was published in the hope that it would stimulate more research into the fragile X chromosome, both into laboratory methods to detect it and into its full clinical significance.
No slight to Dr. Daker and his colleagues was intended. Their report of two normal fragile X males was the first such report and came at a time when the relationship between the fragile X in males and mental retardation appeared to have been established. Naturally this challenge to what had become accepted met with some skepticism. This skepticism may have been increased by the fact that their report was very brief, contained no details of the lymphocyte culture methods used except for the proportion of serum in an unspecified culture medium, and had no psychometric data other than a clinical impression and no further family studies than those on the brother of the index patient. We were correct in stating that the general reaction of the workshop to their report was one of considerable doubt.
Like Dr. Daker, we and most probably others who attended the workshop have been impressed by an increasing number of reports of allegedly normal males with the fragile X and would now have much less doubt about the original report of Dr. Daker and his colleagues on this matter. It certainly now appears likely that some fragile X males do have normal intelligence although such males must be very uncommon. Nevertheless, there are still no well-documented reports of a normal male with the fragile X chromosome. Information that would be most useful when reporting unaffected fragile X males would include detailed information on their mode of ascertainment, family pedigree, and quantitative intelligence and behavioral test data on the unaffected fragile X male of each family and his chromosomally normal sibs.
Prenatal detection of the fragile X chromosome is now feasible. Does this create a new dilemma? Which of the fragile X male fetuses is destined to be retarded, and which is programmed to be of normal intelligence?