Retinoblastoma is a malignant tumor of the retina that occurs primarily in young children as a result of mutations in the retinoblastoma gene (RB), the first tumor suppressor gene to be identified. In about 35% to 40% of patients with retinoblastoma, an RB gene mutation is present in the germline, resulting in hereditary transmission of the disease. Most families with hereditary retinoblastoma demonstrate autosomal dominant inheritance with almost complete penetrance and high expressivity. However, some families display an inheritance pattern characterized by reduced penetrance and expressivity. Recent advances in our understanding of the structure and function of the retinoblastoma protein (pRB) now provide new insights into the molecular basis of this low-penetrance form of retinoblastoma. Low-penetrance retinoblastoma mutations either cause a reduction in the amount of normal pRB that is produced (class 1 mutations) or result in a partially functional mutant pRB (class 2 mutations).