PADI4 and malignant glioma: Increased PAD4 staining has been observed in undescribed astrocytomas [32], while upregulation of PAD2 and PAD3 via cAMP-PKA (cyclic adenosine monophosphate- protein kinase A) signaling has been shown in U251MG astrocytoma cells [33] and PAD-upregulation was shown in response to hypoxia in malignant gliomas [34].