The primary genetic cancer predisposing event in many Li-Fraumeni syndrome families is a germline mutation in the p53 gene. We describe an extended Li-Fraumeni family with a germline mutation in the p53 gene involving a deletion of exon 10. The mutation is a 2.35 kilobase intragenic deletion encompassing exon 10, which results in the specific loss of the entire p53 oligomerization domain. This mutation segregates with the cancer phenotype. A lymphoblastoid cell line developed from a mutation carrier shows accumulation of mutant p53 protein by immunoblotting. However, tumor tissues from two affected carriers are negative by immunohistochemical staining. A major structural alteration specifically involving the oligomerization domain of a germline p53 gene has not been previously described and occurs in a region rarely mutated in sporadic tumors. The oligomerization domain is dispensable for many wild-type p53 functions, including transactivation, sequence-specific DNA binding, and suppression of oncogenic transformation. However, the domain appears to be required for transcriptional repression, and DNA strand reassociation. The identification of this mutation in an LFS family may yield insights into the importance of the oligomerization domain for suppressor function of the p53 tumor suppressor gene.