As the name implies, Rb is a tumour of retinal cells. With
5 years, the majority of tumours occurring before 2 years of age. Individuals can present with Rb at birth, which suggests that the tumours have been growing since early fetal life. This view is supported by the histopathology of the tumour, which shows a relatively undifferentiated, embryonic-like organization, implying arrest in the development of a retinal precursor cell. Thus, clones of cells are arrested in a state in which further genetic changes can occur, thus giving rise to the full tumour phenotype. The exact identity of these precursor cells, however, remains unknown.