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Fig. 8 Model of posterior Hox regulation of tbxta expression. Tbxta and Wnt exist in a positive autoregulatory loop, with Tbxta activating wnt3a and wnt8 expression directly (Martin and Kimelman, 2008). We propose that Wnts activate tbxta expression during the somitogenesis stages through the Tcf7/Lef1 (Tcf) sites in Hox element 1 and 2, located far upstream from the tbxta transcription start site, in collaboration with Hox proteins. Although our data specifically show binding of Hoxa13b to these elements, we suggest that all posterior Hox genes (Hox9-13) act through these elements. In addition, Rbpj (Rp) binds to these elements to aid transcriptional activation. In the notochord, the upstream elements are not necessary for expression during the somitogenesis stages. β indicates β-catenin, which is stabilized by Wnt signaling.