Fig. 7. Model for the method by which vhnf1 functions to generate rhombomere-specific identity. (A) Summary of the data. rX represents the combined r5+r6 domain, a partially differentiated rhombomere that develops in val mutants. rX identity is distinct from rP, the unspecified tissue that remains when Fgf signals are reduced. (B) Model of the role of vhnf1 in generation of rhombomere identity. After hoxb1a, fgf8 and vhnf1 gene expression is established, vhnf1 functions to repress hoxb1a throughout the domain in which vhnf1 is expressed. In addition, Fgf signals from the anterior are received by posterior hindbrain cells, and the combination of Fgf signal transduction and vhnf1 function result in activation of val expression within the domain in which the overlap occurs. val function is then sufficient to activate krox20 expression, although this activation is limited to the presumptive r5 domain, suggesting that other factors limit this activation outside of r5 or promote it within r5.