Retinoblastoma is a form of cancer that occurs in the retina of the eye in young children. It occurs most commonly in children under the age of five. There are two forms of this cancer, a heritable and a non-heritable form. Where the heritable form exists, a tumour can occur in both eyes or less commonly only one eye, on the other hand in the non-heritable form, the tumour only ever exists in one eye only  
- ↑ http://www.macmillan.org.uk/Cancerinformation/Cancertypes/Childrenscancers/Typesofchildrenscancers/Retinoblastoma.aspx
- ↑ Oxford University Press, Cancer in Children:Clinical Management, 2005.