The protein level and activity of critical regulatory proteins in cells are tightly controlled by several tiers of post-translational modifications. HIF-1α is maintained at low levels under normoxia by the collaboration between PHD proteins and the VHL-containing E3 ubiquitin ligase complex. We recently identified a new physiologically relevant mechanism that regulates HIF-1α stability in the nucleus in response to cellular oxygen levels. This mechanism is based on the collaboration between the SET7/9 methyltransferase and the LSD1 demethylase. SET7/9 adds a methyl group to HIF-1α, which triggers degradation of the protein by the ubiquitin-proteasome system, whereas LSD1 removes the methyl group, leading to stabilization of HIF-1α under hypoxia. In cells from a knock-in mouse model that harbor a mutation preventing HIF-1α methylation (Hif1αKA/KA), HIF-1α levels increased in both normoxic and hypoxic conditions. With increased HIF-1α, Hif1αKA/KA knock-in mice displayed increased hematological parameters, such as red blood cell count and hemoglobin concentrations. In pathological aspects, retinal and tumor-associated angiogenesis as well as tumor growth were increased in Hif1αKA/KA knock-in mice. Certain human cancer cells exhibit mutations that cause defects in HIF-1α methylation. In summary, this newly identified methylation-based regulation of HIF-1α stability constitutes another layer of regulation independent of previously identified mechanisms.