Proposed models for the action of Tutl. The homophilic binding between two Tutl proteins on opposing cell surface may be mediated by one or more of its Ig domains. Tutl may mediate homotypic repulsion. Alternatively, Tutl may mediate de-adhesion by antagonizing the function of certain R7-specific cell adhesion molecules. Circles, Ig domains; Rectangles, fibronectin type-III repeats.