Based on previous findings of the hypoxia-induced expression of PAD2 and citrullinated glial fibrillary acidic protein (GFAP) in astrocytes and that of PAD4 in different tissues and biofluids of malignant tumors [39,40], Sase et al. studied the expression of PAD enzymes and protein citrullination in U-251 MG human cells of malignant glioma under normoxia and hypoxia via quantitative polymerase chain reaction (qPCR) and two-dimensional electrophoresis (2-DE) mass spectrometry proteomics [46]. Here, PADI4 is linked to cancer.