We found that neuroblastomas arising in the nf1a+/- and nf1a-/- background exhibited a progressively higher percentage of nuclei with PCNA staining (30% for nf1a+/- and 80% for nf1a-/-) compared to PCNA staining of 20% of nuclei in the wild-type nf1a+/+ genotype (Figure 5). This evidence concerns the gene PCNA and neuroblastoma.