An exhaustive study of erythropoietic protoporphyria in the Netherlands led to the discovery of 200 patients with this disorder in 91 families. In 46 of these families a single patient occurred. A study of parents, sibs and children led to the conclusion that the disease of erythropoietic protoporphyria is inherited as an autosomal recessive disorder. The presence of an occasional fluorescent red blood cell combined with normal protoporphyrin levels was observed in half of the children and sibs of patients and one of their parents; this phenomenon is therefore inherited as an autosomal dominant character. From an analysis of the findings in the 91 families we put forward the hypothesis of a 3-allele system.

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In an ancient fishing village in the neighbourhood of Leiden a cluster of melanoma-prone families was detected. In these families atypical precursor naevi were recognizable in many melanoma patients and in their relatives. This combination of symptoms is called the Dysplastic Naevus Syndrome (DNS). The spectrum of clinical signs characterizing the phenotype of the DNS ranged from apparent lack of gene-expression through minimal to obvious manifestations. Genetic analysis of six pedigrees revealed an autosomal dominant mode of inheritance, with a very variable expressivity of the gene, which is also rather frequently non-penetrant. The total number of family members (affected or at risk) was 314, 243 of which were alive and were personally examined by us. In the six pedigrees a total of 33 patients with melanoma occurred. In thirty patients with multiple atypical naevi, the clinical diagnosis of DNS was histologically verified. A further thirty-six patients with several atypical naevi were also regarded as gene carriers. Fifteen 'normal' individuals had to be gene-carriers on the basis of their position in the pedigree.

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