Fig. 10. A schematic model for the regulation of Krox20 expression in r5-derived neural crest. We propose that the establishment of Krox20 expression in r5-derived neural crest involves two steps. Krox20 is first activated in the entire r5 territory under the control of an initiator cis-acting element distinct from the NCE (large grey arrow). In r5 premigratory NC cells, the Krox20 protein and crest-specific Sox proteins bind to sequences in the NCE and co-operate in the establishment of a positive, autoregulatory loop (large black arrow). This autoregulatory mechanism is then sufficient to maintain Krox20 expression in migratory NC cells. K1 and K2, Krox20 binding sites; HMG1/2, putative Sox protein binding sites.