A 2 year old girl is evaluated because of deteriorating vision. On physical examination, a whitish hue is noted in the right pupil, along with strabismus. Palpation of the eye elicits apparent pain and tenderness. The patient is referred to an ophthalmologist, who schedules surgery, and removes a multifocal tumor from the right eye. The pathology repo notes the presence of rosettes composed of cuboidal-to-columnar cells surrounding a central lumen in the tumor. In all likelihood, this child's condition stems from a mutation in a gene on chromosome ?

The description of this case is classic for retinoblastoma, the most common malignant eye tumor of childhood. Retinoblastoma presents as in the question, and microscopically is composed of neuroepithelial cells that form characteristic rosettes (Flexner-Wintersteiner rosettes). Familial retinoblastoma is often bilateral or multifocal, as in this case. A pre existing mutation of a tumor suppressor gene, Rb, located on chromosome 13 (13q14) is present, but causes no symptoms. A second, somatic mutation in a retinal cell leads to loss of a nuclear protein that prevents the retinal cell from exiting G1, allowing development of the tumor. In cases of sporadic retinoblastoma, mutations must occur in both chromosomes 13 of a somatic cell, a rare event, hence sporadic retinoblastoma is always unifocal and unilateral. Ref: Meric-Bernstam F., Pollock R.E. (2010). Chapter 10. Oncology. In F.C. Brunicardi, D.K. Andersen, T.R. Billiar, D.L. Dunn, J.G. Hunter, J.B. Matthews, R.E. Pollock (Eds), Schwaz's Principles of Surgery, 9e