The autosomal dominant multiple endocrine neoplasia type 1 (MEN1) syndrome is characterized by neoplasia of parathyroids, anterior pituitary, and gastrointestinal and pancreatic neuroendocrine tissues. Recently the gene responsible for the MEN1 syndrome has been identified on chromosome region 11q13. Most of the described mutations are nucleotide substitutions and small deletions affecting exons 2 and 3, causing protein truncation. Only one mutation in exon 5 has been found, and this corresponds to a MEN1 sporadic case. Small insertions are also rare. We studied a MENI family composed of five members, two of whom were clinically affected. We found a new germline 1 basepair insertional mutation affecting the exon 5 of the MEN1 gene in the two members affected in this MEN1 family.