As development proceeds, cells acquire new fates so that they can carry out their proper function within the organism. Conventionally, cell fate specification has been thought of in terms of the activation of transcription factors that subsequently activate cell type specific genes. In this proposal we examine a role for a winged-helix transcriptional repressor, foxd3, in zebrafish neural crest cell fate choice. We hypothesize that foxd3 is involved in cell fate choice not by activating genes for particular cell types but instead by preventing expression of genes involved in acquisition of other fates. We propose that foxd3 specifically represses the bHLH transcription factor mitfa, which is necessary for melanoblast specification. We will address this hypothesis with the following aims: 1. We will test the hypothesis that mitfa is transiently transcribed in glial-pigment cell progenitors, and subsequently repressed in non-melanogenic cells. We will compare the expression of pigment cell markers to the expression of GFP mRNA driven by the mitfa promoter in transgenic animals, and will construct a transgenic line using a fluorescent reporter that acts as a timer to distinguish when transcription and translation occur. 2. We will test the hypothesis that foxd3 represses mitfa expression. We propose to: identify foxd3 binding sites in mitfa promoter, and test whether foxd3 and mitfa are co-expressed. 3. We will test the hypothesis that foxd3 regulates neural crest cell fate decisions by regulating mitfa. We will test the effects of foxd3 on neural crest cell fates by overexpression or antisense morpholino knockdown within the whole embryo and within individual neural crest cells, and test foxd3 loss/gain of function in the context of the mitfa and c-kit mutant backgrounds. These experiments will shed light on the differentiation of a possible common glial/melanoblast precursor and lead to understanding how abnormal development of this precursor might underlie congenital tumors.