We agree with Professor Emery that evidence is adding up suggesting heterogeneity of DMD at the molecular and cytogenetic levels Poyd and Buckle, 19861. However, this does not seem evident on clinical grounds.
Our group of patients classified as mentally retarded [Bortolini and Zatz, 19861 all had an IQ < 50 and therefore is comparable to that studied by Emery et a1 [1979].
These affected boys did become confined to a wheelchair significantly later than boys with normal intelligence. However, they started to walk later also, and when the total duration of their walking period were compared, no significant difference was observed between the patients with normal IQ and those who were mentally retarded.
Among the 98 patients we had analyzed, 23 have already died. Although our assumptions are still based on a small sample, when we compared the period of confinement to a wheelchair until death, the differences between norma1 (7), borderline (1 l), and severely handicapped (5) were not statistically significant.
Therefore, we think that storing DNA from members of informative families and in particular from those with severely mentally retarded patients is going to be very important to solve this issue in the near future.