RB1 and retinoblastoma: Despite biallelic mutation of RB1 existing in nearly all retinoblastomas, a subset of RB tumors (approximate ~1.4%) reveal no evidence of mutated RB1 gene [18, 19] and possess high-level amplification of oncogene MYCN. Besides, this kind of amplified MYCN tumors have diverse morphology and patterns of gene expression different from RB1−/− tumors, indicating a unique subtype [20].