Although most mutations in cancer cells are somatic, about 1% of all cancers are inherited (germ line cancer). In these cases, the mutation is carried in the genetic code of each cell in the body. Families with inherited cancer syndromes generally display cancer in an autosomal dominant pattern in two or more generations. More than 20 types of inherited cancer syndromes have been identified (see box on page 29 ) ( Hill, 2001 ; Rettig & Sawicki, 2001 ). Most of these tumors are caused by tumor-suppressor genes, although some are caused by oncogenes. Other tumors arise from mutations in DNA repair genes (e.g., hereditary breast cancer, hereditary nonpolyposis colorectal cancer). Inheriting one mutated gene is usually not enough to cause cancer, although it is sufficient to transmit a trait or characteristic (phenotype).