Although it is well known that patterning in the retina of vertebrates is essential for retina formation and for the retinotopic projection of axons in the embryo, knowledge of molecular and cellular mechanisms of retina patterning is limited. We have previously identified the Xenopus Tbx3 gene (XTbx3) which is expressed in the dorsal retina but not in the ventral retina in Xenopus embryos [H. Li, C. Tierney, L. Wen, J. Y. Wu, and Y. Rao (1997) Development 124, 603-615; M.-L. He, L. Wen, C. E. Campbell, J. Y. Wu, and Y. Rao (1999) Proc. Natl. Acad. Sci. USA 96, 10212-10217]. Dosage-sensitive phenotypes in humans suggest that the manipulation of the amount and location of its products could be informative for understanding its normal function. Here we report that ectopic expression of Tbx3 by mRNA injection suppressed formation of the ventral retina. Furthermore, Tbx3 injection led to inhibition of molecular markers for the ventral retina including Pax-2 and netrin, indicating that Tbx3 plays an important role in retina dorsal/ventral patterning in vertebrates by inhibition of gene expression for ventral retina specification.