Fig. 1. A transposon-mediated gene transfer system that enables conditional gene expression in the developing chicken retina. (A) Schematic drawing of conditional gene expression. The gene cassettes flanked by the Tol2 cis-sequence (Tol2) are integrated into the host genome by the transient expression of Tol2 transposase (T2TP). Only the cells that stably incorporated both CAG-rtTA-M2 and TRE-mEGFP cassettes express mEGFP upon the addition of doxycycline (DOX), which enhances the binding of rtTA-M2 to its cis-element TRE. (B,C) Exogenous mEGFP expression (green) in horizontal cells expressing Prox1 (B) and Pax6 (C) (magenta). Scale bars: 20 µm.