Editor—Progressive diaphyseal dysplasia (MIM 131300) is an autosomal dominant osteochondrodysplasia with late onset and marked variability in expression of clinical and radiological features.1 We have recently described a patient with this diagnosis and suggested the presence of anticipation,2 the tendency of a familial disorder to occur earlier in the younger than in the older generations of a family.3

In order to assess whether this hypothesis were true, we examined 24 other people from the same family (fig 1) who were invited and agreed to participate. We excluded the proband and tried to avoid ascertainment bias by including in the study all family members who were alive in four generations. Although the diagnosis of progressive diaphyseal dysplasia had not been previously recorded in any of them, some family members had assigned to themselves the status of affected or unaffected. This was often discordant with the clinical and radiological data. It is also likely that people with bone complaints were more eager to consent to …