Immunofluorescence image of human carcinoma cells retrovirally infected with a large combinatorial library of artificial transcription factors, composed of optimized zinc finger DNA-binding domains (yellow, blue and pink zinc finger structures) shuffled and fused to a transcription activator domain (blue spheres). Infected cells co-expressing GFP (intracellular green) and showing activation of the normally silent Ve-cadherin cell surface marker gene (orange staining) were sorted by FACS (Fluorescence Activated Cell Sorting) for the isolation of new functional Transcription Factors.