One of the major components of the cellular response to oxygen deprivation is a heterodime- ric transcription factor - hypoxia inducible factor-1 (HIF-1). HIF-1 is constitutively produced and de- graded by the ubiquitin-proteasome system under normoxic conditions but becomes stabilized and transcriptionally active under hypoxic conditions. Hypoxia is one of the typical features of growing tumor and enhanced activity of HIF-1 observed in several different malignancies, is one of the most important factors responsible for activating pathways leading to tumor angiogenesis. Therefore better understanding of HIF-1 function may provide new targets for tumor therapy.