Rex-1 (for reduced expression), also designated zinc finger protein 42 (ZFP42), is an acidic zinc finger protein. Rex-1 contains four repeats of the zinc finger nucleic acid-binding motif and a potential acidic activator domain, suggesting that it is a regulatory protein. Rex-1 localizes to the nucleus and is highly expressed in embryonic stem (ES) and undifferentiated murine F9 teratocarcinoma cells. At the transcriptional level, expression of Rex-1 is reduced when F9 cells are induced to differentiate by the addition of retinoic acid (RA), and Rex-1 repression is enhanced by E1A. The Oct-3/4 transcription factor can either activate or repress the Rex-1 promoter, depending on the cellular environment, while Oct-6 can lower the expression of Rex-1.