RB1 and retinoblastoma: Therefore, in the presence of a partially functional RB1 protein, the precursor cells form a retinocytoma instead of a retinoblastoma (Harbour, 2001; Lohmann et al., 1994; Otterson, Chen, Coxon, Khleif, & Kaye, 1997; Sakai, Ohtani, McGee, Robbins, & Dryja, 1991).