GDNF and coloboma: Secondly, the marked, strong coloboma phenotypes can be observed in both TGFβ2−/− GDNF−/− and TGFβ2−/− GDNF+/+ embryos from a mixed breeding background, occasionally also presenting a dorsal coloboma, while the phenotype of TGFβ2 KO embryos from a sole breeding background is noticeably milder (figure 1a–d, electronic supplementary material, S1B–C).