['To uncover the mechanism by which hypoxia enhances cartilage matrix synthesis by human articular chondrocytes.', 'The hypoxic response was investigated by exposing normal (nonarthritic) human articular chondrocyte cultures to 20% oxygen and 1% oxygen. Induction of the differentiated phenotype was confirmed at the gene and protein levels. In its first reported application in human articular chondrocytes, the RNA interference method was used to directly investigate the role of specific transcription factors in this process. Small interfering RNA directed against hypoxia-inducible factor 1alpha (HIF-1alpha), HIF-2alpha, and SOX9 were delivered by lipid-based transfection of primary and passaged human articular chondrocytes. The effect of each knockdown on hypoxic induction of the chondrocyte phenotype was assessed.', 'Hypoxia enhanced matrix synthesis and SOX9 expression of human articular chondrocytes at both the gene and protein levels. Although HIF-1alpha knockdown had no effect, depletion of HIF-2alpha abolished this hypoxic induction. Thus, we provide the first evidence that HIF-2alpha, but not HIF-1alpha, is essential for hypoxic induction of the human articular chondrocyte phenotype. In addition, depletion of SOX9 prevented hypoxic induction of matrix genes, indicating that the latter are not direct HIF targets but are up-regulated by hypoxia via SOX9.'] Is hypoxia-inducible factor 2alpha essential for hypoxic induction of the human articular chondrocyte phenotype?