Prolyl-4-Hydroxylase Domain (PHD) 1-3 are the cellular sensors for mediating oxygen-dependent gene expression via regulation of the hypoxia-inducible factor (HIF)-1a and 2a. They are members of the Fe(II) and 2-oxoglutarate (2OG) dependent oxygenase family. Besides similarities in the hydroxylation reaction in vitro, the three PHDs differ in their ability to hydroxylate HIF-1a in vivo with PHD2 as the main HIF-1a?modulating enzyme. Whereas PHD2 is widely expressed in most tissues, PHD1 and PHD3 show a more restricted organ-specific expression pattern in testis, skeletal muscle or heart tissue, respectively. Searching for new PHD protein interaction partners using the yeast two hybrid screen technology, we identified the activating transcription factor-4 (ATF-4) as new potential substrate of PHD3, whereas PHD1 and PHD2 did not affect ATF- 4 stability. To further analyze PHD3-specific function we established stable transfected PHD3 knock down HeLa cell clones. Taken collectively, there is accumulating evidence, that the three PHD isoforms have non-redundant specific functions regarding expression pattern, regulation of the HIF system and other PHD-specific mediated pathways.