['Amongst different stem cell populations derived from human cord blood (CB), unrestricted somatic stem cells (USSC) are distinguished from CB mesenchymal stromal cells (CB MSC) by expression patterns of homeobox (HOX) genes, delta-like1 homolog (DLK1) expression and adipogenic differentiation potential. In this study we investigated the effects of oxygen tension on the generation, proliferation and expression of stem cell marker genes, which could be critical during large-scale cell culture for clinical applications.', 'We cultured CB-derived stem cells at 5% and 20% O(2). Telomere length shortening was analyzed and we investigated gene expression using reverse-transcription (RT)-polymerase chain reaction (PCR) and real-time PCR. Additionally we performed adipogenic and osteogenic in vitro differentiation. Results. Altering the cultivation conditions of USSC or CB MSC from 20% to 5% O(2) had no significant impact. In contrast, cell populations derived from primary cultures prepared at 5% O(2) qualified as neither USSC nor as CB MSC. When converted to 20%, their proliferation was diminished, telomere shortening was accelerated, and two of six cell lines ceased expression of HOX genes. The HOX code of the other cell populations was not been affected by culture conditions.'] Does oxygen tension modify the 'stemness ' of human cord blood-derived stem cells?