The hypoxic microenvironment of a growing tumor may then trigger an oxygen (O2)-sensitive transcriptional program in tumor cells by activating two basic helix-loop-helix transcription factors, i.e., the hypoxia-inducible factors HIF-1 and HIF-2, which drive the expression of a myriad of growth factors and cytokines [54]. The gene discussed is HIF1A; the disease is neoplasm.