Medical Genetics The mutations occur after birth and result in tumor formation Tumors usually develop in only one eye and patients are not at high risk for other cancers both alleles need to be mutated in a single cell, and that is why this form typically occurs only in one eye.Medical Genetics The mutations occur after birth and result in tumor formation. Tumors usually develop in only one eye and patients are not at high risk for other cancers. Both alleles need to be mutated in a single cell, and that is why this form typically occurs only in one eye