Abstract: Master regulators can profoundly alter the transcriptional dynamics of a cell to the extent that the cell adopts a new cell fate. For example, transcription factors, Oct4, Sox2, Klf4, and c Myc, can reprogram a differentiated cell type to an induced pluripotent stem (iPS) cell state. With the right cocktail of transcription factors or small molecules that alter transcription, one can also differentiate pluripotent cells or progenitors into a particular cell type with a specified function. However, the process of discovering master regulators or combinations of factors that control cell fate choices is challenging because testing natural transcription factors by trial and error is often labor-intensive and cost-prohibitive. Moreover, transcription factors function in a specific cellular milieu and trigger appropriate gene expression in response to specific cues that might not be reproduced in cellular systems where they are being tested. We developed a library of gene-activating artificial transcription factors (ATFs) with the complexity to target an array of 9-bp sequences in the genome. We used this ATF library to screen for factors that can turn on an Oct4 reporter gene in mouse embryonic fibroblasts and convert them into iPS cells. We identified combinations of ATFs that can activate the pluripotency network without exogenous expression of Oct4. In addition, the ATF library can serve as a versatile tool to perturb transcriptional networks that differentiate ES cells into blood and heart lineages. This forward genetics approach provides a powerful means to identify master regulators in an unbiased manner.