Retinoblastoma is a rare malignant tumor which usually appears in infants or young children. It occurs at a frequency of about one in every 15,000 births. In some cases, there is a family (familial) history of the disease.

The genetic cause of retinoblastoma has been extensively studied. It is described as a "two-hit" process. Normally, individuals have two good copies of the retinoblastoma gene (RB-1) on chromosome 13. The disease develops in individuals in which mutation has occurred in both copies of RB-1. It appears that about 40% of patients are born with a defective copy (first "hit"), inherited from one parent. The second copy is rendered defective by a separate mutation (second "hit") that occurs in the eye. Individuals with an inherited RB-1 defect have high likelihood of developing retinoblastoma in both eyes. For these individuals, diagnosis usually occurs by age one. These patients also have increased risk of developing...