To further investigate the causes of cell proliferation decline under hypoxia, the protein immune blot after lysating cells showed that, the proliferating cell nuclear antigen (PCNA) expression level in the NOX4 knockdown cells under hypoxia was downregulated (Fig. 1D,E), highlighting the effect of NOX4 in regulating the growth of thyroid cancer cells under hypoxic microenvironment. Here, NOX4 is linked to thyroid cancer.