Familial adenomatous polyposis (FAP) is an autosomal dominant disorder related to germline mutations of the adenomatous polyposis coli ( APC) gene. It is characterized by the detection of numerous adenomatous polyps that, if untreated, develop into colorectal cancer. We studied an Italian family with FAP history and the related colorectal tumor sample of the proband. Sequencing analysis of blood samples revealed the presence of a never-reported germline mutation in the APC gene (exon 15): an heterozygous G deletion at position c.2126 resulting in a premature stop codon (p.Gly721GlufsX6) and in a truncated protein. This mutation was also identified in the colorectal tumor tissue, together with a second known pathogenic heterozygotic somatic mutation, c.4348C>T (p.Arg1450X), which generates a premature truncated protein. The novel identified germline mutation is therefore related to FAP and, in accordance with Knudson's “two hit” hypothesis, can be considered the first event predisposing to the insurgence of colorectal cancer in these patients. The somatic hit inactivating the second allele of the APC gene is located in the mutation cluster region of the gene; this is not a random event since it depends on the position of the germline mutation. The inactivation of APC generates the neoplastic growth advantage to the cell.