Fig. 8. The ability of Id3 to block the differentiation of neural crest derivatives is reversible. Control embryos at stage 40 (A) display substantially more melanocytes than do sibling embryos injected with high doses of Id3 (B). In marked contrast, when embryos are injected with lower doses of Id3 in order to allow cells to recover from the effects of Id3 at an earlier time point, a profound increase in the numbers of melanocytes is observed (C). These results demonstrate that although forced expression of Id3 maintains neural crest cell in a progenitor state for a prolonged period of time, it does not irreversibly alter their ability to give rise to neural crest derivatives.