NFIB and neoplasm: By contrast, the EGFP+ sympathoadrenal cells in the IRG of nf1a-/- homozygous zebrafish expanded much more rapidly, with an average 6.61-fold expansion rate within the 3 week period (nf1a-/-;nf1b+/+; MYCN;EGFP) and tumor development in 9 of 10 fish before 6 weeks of age (Figure 4B).