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Genes have also been found for several types of cancer that can run in families. Several of these are rare conditions that affect only a few people: a childhood eye cancer known as retinoblastoma; Wilms' tumor, a kidney cancer that usually appears before age 5; and the Li-Fraumeni syndrome, in which children and young adults of the family develop an assortment of cancers, including sarcomas in the bones and soft tissues of the arms and legs, brain tumors, acute leukemia, and breast cancer. In 1993, scientists identified the gene that causes familial adenomatous polyposis, an inherited predisposition to form precancerous polyps. This condition is believed to be responsible for about 1 percent of colon cancers. More recently, scientists have identified gene mutations that are linked to inherited tendencies toward common cancers, including colon cancer and breast cancer. Families who carry these altered genes may also have an increased risk of other cancers. Women with an altered copy of the BRCA1 breast cancer susceptibility gene, in particular, are susceptible to ovarian cancer as well. People who inherit cancer genes are more likely to develop cancer at a young age, because the predisposing gene damage is present throughout their lives, ready to set cancer's uncontrolled growth in motion should the normal allele be lost or inactivated. Such inherited, or familial, forms of cancer represent perhaps about 5 to 10 percent of all cancers. The great majority of people who get breast cancer or colon cancer have not inherited such highly active altered genes. This is true even for many families that have several members with cancer; certain cancers are so common that some clusters are bound to happen purely by chance. Cases that are diagnosed at older ages, in particular, are more likely to be caused by acquired mutations. Nevertheless, because breast and colon cancer are so widespread, even a small fraction of the total equals a very large number. It is estimated that as many as 1 in 300 women may carry inherited mutations of breast cancer susceptibility genes, and approximately the same proportion of Americans carry mutations that make them susceptible to colon cancer.