NFIB and neoplasm: Expansion of EGFP+ sympathoadrenal cells was observed in the IRG of nf1a heterozygous zebrafish (nf1a+/-;nf1b+/+;MYCN;EGFP, Figure 4A middle panel), with an average increased expansion of 1.73 fold by the end of the 3-week period and a tumor induction rate of 43% (9/21 fish had tumors before they reached 6 weeks of age; Figure 4B).