The rapid growth of the primary tumor is not sufficiently supported by a concomitant development of the vascular system; this determines a reduction of oxygen partial pressure in the tumor microenvironment, and the stabilization of the transcription factor HIF-1α (Hypoxia Inducible Factor-1α), that, accumulated in the nucleus, will form a stable complex with the HIF-1β subunit. The gene discussed is HIF1A; the disease is neoplasm.