Practice question

Six3, a homeobox transcription factor expressed early in anterior neural plate and developing forebrain, cooperates with Pax6 to establish eye lineage. It binds directly to Pax6 ectoderm enhancer and Sox2 regulatory regions, maintaining their transcription in preplacodal ectoderm. Six3 mutants show downregulation of Pax6 and complete absence of lens and retina. Misexpression expands Pax6 domain and induces ectopic lenses in chick. Beyond maintenance, Six3 together with Pax6 recruits coactivators to activate downstream lens genes, positioning it at top of retinal and lens specification network with essential cooperative function.