Fig. 10. Model of endoderm formation in zebrafish incorporating the results of our study [modified from Alexander et al. (Alexander et al, 1999), Reiter et al. (Reiter et al., 2001) and Aoki et al. (Aoki et al., 2002)]. Nodal ligands, together with TARAM-A and Oep, induce the expression of mezzo, bon/mixer, faust/gata5 and casanova in a direct manner. Maintenance of mezzo expression requires the activity of bon and faust (broken lines) but not cas. Other cross-regulatory interactions are not known. mezzo, bon/mixer and faust/gata5 cooperate to induce sox17 by establishing a stable expression of casanova, which is the main determinant of sox17 expression.