Method of diagnosing juvenile polyposis (JP)

Familial juvenile polyposis is an autosomal dominant disease characterized by a predisposition to hamartomatous polyps and gastrointestinal cancer. The present invention shows that JP families carry germline mutations in SMAD4/DPC4, a gene on chromosome 18q21.1. The mutant SMAD4 proteins are truncated at the carboxyl-terminus and lack sequences required for normal function. Methods and compositions for the detection and amelioration of FJP and gastrointestinal tumors are provided.