1. The result of segregation analysis is compatible with the hypothesis of a simple autosomal recessive mode of inheritance. 2. The number of sporadic cases is not greater than expected. 3. Among the affected siblings in the sibships analyzed, males are somewhat more frequent than females. However, this sex difference is also found among the unaffected siblings, and it is not statistically significant. 4. Contrary to assertions made in the literature, there is no clustering of affected in the sequence of siblings, no maternal age effect, and no preference of higher birth orders. 5. A high intrafamilial correlation for age at onset, and (very probably) number and severity of malformations points to genetic heterogeneity. Apart from the standard type, an especially mild type with late onset, few malformations, and a relatively benign course seems to exist. Its counterpart is possibly an especially severe type with early onset, many malformations, and a malignant course. However, definite conclusions on the special character of this heterogeneity will require application of additional methods.