Fig. 4. Noggin but not Otx2 regulates eye field transcription factor expression while Otx2 blocks the repression of ET by noggin. (A) RT-PCR was used to detect changes in the expression of the EFTFs in response to noggin (10 pg) and Otx2 (200 pg). The effect of Otx2 on its own expression was not determined (ND). The presence of ET and Six3 in uninjected animal caps (U) was not a result of DNA contamination as neither transcript was detected when duplicate samples were amplified in the absence of reverse transcriptase (-RT). Uninjected sibling embryos `E' were used as a positive control for PCR. Histone H4 was used as a loading control. (B) RT-PCR was used to determine the relative expression of ET in ectodermal explants from embryos injected with Otx2, noggin or both. The percent of ET expression relative to uninjected controls is shown above each bar of the graph. (C) Interpretation of the combined results from A and B.