Germline MEN1 gene mutations are responsible for multiple endocrine neoplasia type 1 (MEN1), a dominantly inherited cancer syndrome. We identified a MEN1 germline mutation 894-9 G --> A in three MEN1 patients from two unrelated families. This mutation was not present in any of the 100 blood samples from normal volunteers. The wild type MEN1 sequence was lost in the patient's pancreatic tumor. Abnormal mRNA was identified in the tumor, which retained an intronic sequence indicating aberrant mRNA splicing at a newly created splicing acceptor site. These findings indicate that this nucleotide substitution is, though previously reported to be a polymorphism, a causative splicing mutation.