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Fig. 8. Model for the mechanism whereby RA signaling establishes the posterior limit of the amphioxus pharynx. RA signaling via heterodimers of RAR/RXR in the posterior fore/midgut endoderm directly activates Hox1. RAR probably autoregulates its own transcription. Hox1 in turn represses expression of the pharyngeal markers Pax1/9 and Otx (black type). Pitx, Notch and Nodal (gray) are also repressed in the posterior fore/midgut by RA signaling and may also be downstream of Hox1.