The effects of hypoxia on resistance of tumors to chemotherapy can be attributed to: a) reduced diffusion of drugs to hypoxic areas; b) decreased proliferation of hypoxic tumor cells due to HIF-1-induced energy-saving pathways; c) selection of inherently resistant cells with mutations in the DNA damage response; d) HIF-1-mediated activation of the DNA repair apparatus, e) HIF-1-triggered induction of genes conferring drug resistance, and f) HIF-1-triggered death of cells unable to adapt to hypoxia. This evidence concerns the gene HIF1A and neoplasm.