Monodactyly is a sign of at least 3 different types of autosomal dominant ectrodactyly. In the first type only the 1st and the 5th or only the 5th toes are present on both feet. The trait is fully expressed in all affected children of patients. No skipping of a generation has been observed. Both parents of several affected children may be normal, or one parent may show minor manifestations only. Single strand mutation is suggested as an explanation of these exceptional cases. Monodactyly is seen less frequently in the second type, ectrodactyly, ectodermal dysplasia and cleft lip and palate (the EEC syndrome), than in the first type. The limb defects are more variable. The third type of ectrodactyly shows extreme intrafamilial variability, comprising various degrees of ectrodactyly, monodactyly and adactyly, defects of the ulna and/or of the tibia. Minor manifestations often occur in affected children of patients. Skipping of a generation is not uncommon.