This chapter summarizes the characterization of the function of Multiple Endocrine Neoplasia type 1 (MEN1) as well as a review of mutations in the MEN1 gene. In 1978, A. jJ. Knudson proposed that the second hit could involve a loss of genetic material, which was first proven by in retinoblastoma. The MEN1 gene contains 10 exons, but only exons 2–9 are transcribed encoding a 610-amino acid protein product which has an estimated weight of 67 kDa. The first exon is noncoding and constitutes most of the 111 nucleotide 5' untranslated region. The exon sizes range from 41–1296 nucleotides and the introns from 79–1563 nucleotides. It has no homology to any previously known protein, making its function unpredictable However, several lines of research have been undertaken to attempt to elucidate its functional roles. It is obvious that more studies are needed to characterize the functions of the MEN1 gene and its molecular mechanisms in MEN1 tumorigenesis.