Retinoblastoma: A malignant eye tumor usually seen in children, that arises in cells in the developing retina that contain cancer-predisposing mutations in both copies of the gene RB1. Abbreviated RB. The most common sign of RB is a white pupillary reflex to light (leukocoria). There are two forms of RB: hereditary and sporadic. The inherited form of RB is usually present at birth as multiple tumors in both eyes. It is due to the transmission of an RB1 germline mutation followed by an acquired somatic RB1 mutation. The sporadic form of retinoblastoma has later onset and typically leads to a single tumor in only one eye. It is due to acquired mutations in both RB1 genes. Patients with hereditary RB are at increased risk of developing tumors outside the eye, including pinealomas (in the pineal gland of the brain), osteosarcomas, soft tissue sarcomas, and melanomas. When RB is detected at an early stage, it can sometimes be treated locally but often requires removal of the eye (enucleation). Early diagnosis and treatment of RB and RB-related tumors reduces morbidity and increases longevity.