Retinoblastoma (Rb) is initiated by germline mutations in the RB1 gene. Up to date, no mutation was identified in exons 26 and 27. We have identified a 2 bp frameshift insertion in exon 27 of the RB1 gene (RBg.177008_177009dup) in a boy with unilateral Rb and his healthy father that has occurred de novo on the allele transmitted by the father's father. RT-PCR showed that the mutant +2 bp transcript is present in RNA from peripheral leukocytes after short-term culture. The level of the mutant transcript was low compared to the normal transcript indicating abnormal expression of the variant allele. The mutant transcript was further reduced after puromycin treatment suggesting that NMD is not involved. Although oncogenic mutations in the terminal exons of the RB1 gene are rare molecular testing is important as those terminal mutations can be associated with incomplete penetrance and cause high recurrence risk in family members.