Neuroendocrine tumors (NETs) are a heterogeneous group of benign and malignant 
neoplasias, detectable in the context of hereditary tumor syndromes in up to 30% 
of cases. The pathogenic understanding of NETs has increased considerably during 
the last decade, mainly due to the identification of underlying genetic defects 
and the availability of genetically modified animal models. These developments 
are reflected in a revised WHO classification of gastrointestinal NETs. In 
contrast to a variety of rare neuroendocrine tumor syndromes, multiple endocrine 
neoplasia syndrome type 1 (MEN1) and type 2 (MEN2) play clinically significant 
roles due to their common incidence. MEN1 and MEN2 are classic 
autosomal-dominant familial tumor diseases with a high penetrance and variable 
clinical expression, caused by germ line mutations of the MEN1 tumor suppressor 
gene and the RET protooncogene, respectively. The clinical management of 
patients with NETs has changed significantly after the introduction of clinical 
genetic screening. The detection of MEN1 mutations allows for risk-adapted 
treatment and follow-up. RET gene analysis can identify individuals with a very 
high risk to develop familial medullary cancer (MEN2), who may be successfully 
treated by prophylactic thyroidectomy. NETs thus represent a paradigmatic 
example of the successful link between basic genetic science and clinical care 
in molecular medicine.