A: In 15 somite staged embryos, neural crest cells expressing Sox10 migrate out of the neural tube and into the pharyngeal region (asterisks), but do not divide into separate arch populations (asterisk in wild type embryo). After entering the arches, neural crest cells establish a proximal-distal axis through nested and combinatorial expression of Dlx transcription factors (illustration in A). B: In 18 somite staged Foxi3 mutants, neural crest cells adopt the nested expression pattern (A), but the population of cells expressing the intermediate and distal factors Dlx5 and Dlx3 are smaller in the mutant first arch. Scale bars are 100μm.