Forty percent of retinoblastomas are heritable: the child inherits one mutant allele through the germline and a somatic mutation in the other allele occurs, resulting in the loss of function of that gene. This leads to tumor development, often with multiple tumors in both eyes. Penetrance. however, is not complete. since the second mutation is a chance event. Sixty percent of retinoblastomas are sporadic: both alleles are inactivated by somatic mutation. Because this is a rare event, tumors usually are in only one eye and present at a later age. To detect a mutation in the Rb gene and differentiate between heritable and sporadic cases of retinoblastoma, PCR is the most appropriate technique of the choices given. Using the appropriate primers, a deletion can be detected as a change in size of the DNA - band amplified. In heritable cases, one chromosome will show the altered length from the deletion or the translocation in any cell in the body. In sporadic retinoblastoma. samples isolated from anywhere other than the tumor should have two normal alleles.