Gain of function by retroviral gene expression. Viral solution is injected into the anterior neural tube of ED1.5 chicken embryos (A) or in the vitreal cavity of ED3 chicken embryos (B). (C-D) Transversal section of the retina of an ED12 embryo that was injected with RCAS construct expressing GFP on ED3. (C) Shows GFP expression (green) throughout the layers of the retina; (D) detection of viral infection by an antibody against RCAS subgroup A envelop protein (magenta). Abbreviations: gcl: ganglion cell layer; inl: inner nuclear layer; onl: outer nuclear layer.