Zebrafish are capable of regenerating parts of the retina, fins, and heart. Following light-induced destruction of the cone photoreceptors, Müller glia (MG, retinal stem cells) are activated to de-differentiate and then proliferate to produce multipotent retinal progenitor cells. Progenitor cells then migrate into the layer of photoreceptors and differentiate into new cones to replace those destroyed. A comparison of gene expression data from microarray analysis of activated MG isolated from regenerating zebrafish retinas (Z. Qin et al., 2009, PNAS) with microarray expression data from regenerating zebrafish fin and heart tissues identified tgif1 as one of the transcription factor genes up-regulated during regeneration of all three tissues. Human TG-Interacting Factor (TGIF) is a highly conserved transcriptional repressor that negatively regulates TGFbeta signaling by binding to the phosphorylated Smad2-Smad4 complex and recruiting co-repressors. We are working with a tgif1 zebrafish mutant that is predicted to encode a truncated Tgif1 protein (~18 kDa) that should lack the Smad-binding domain; wild-type zebrafish Tgif1 protein is 35 kDa. We hypothesize that: 1) Tgif1 is specifically expressed in MG when they first de-differentiate and begin to proliferate. 2) Zebrafish homozygous for the Tgif1 mutation will express a truncated Tgif1 protein lacking the Smad-binding domain and will be unable to bind Smad and corepressor proteins. 3) Regeneration of cone photoreceptors will be compromised in the mutant due to increased in TGFbeta signaling. Histological analysis confirmed previous microarray data that tgif1 is expressed in MG following light-induced destruction of photoreceptors. Western blots and co-immumoprecipitations were used to characterize the truncated Tgif1 mutant protein and protein-protein interactions. Consistent with our prediction, zebrafish homozygous for the mutant tgif1fh258 allele expressed only the truncated Tgif protein. Co-immunoprecipitations and analysis of Tgif1 expression during cone regeneration are on-going.