HIF1A and cancer: Fourth, the effects of conventional anticancer drugs are attenuated because of the suppression of cancer cell proliferation in the hypoxic microenvironment; congruently, HIF-1 has been shown to negatively regulate cyclin D1 and c-Myc expression and increase p21CIP1 protein levels, thereby halting the progression of the cell cycle and negating the effect of chemotherapeutics [52,53].