HIF1A and neoplasm: However, in ES in vivo expression of HIF-1α and cleaved caspase 3 localised to areas adjacent to necrotic, and presumably hypoxic, tissue and HIF-1α has been correlated with the percentage of dead cells in OS tumours [6]., This suggests some in vivo relationship between HIF and apoptosis which we have been unable to dissect in the in vitro situation.