The PAD (Peptidylarginine Deiminase) family is a group of enzymes that convert arginine residues into citrulline residues, including members such as PAD1, PAD2, PAD3, PAD4, and PAD6. They possess catalytic domains and calcium ion binding sites. PAD proteins are primarily located in the cytoplasm and nucleus, participating in post-translational protein modification, gene expression regulation, and cell signaling. PAD genes are expressed in various cells and tissues, with tissue-specific expression and mutations associated with a variety of diseases, including autoimmune diseases, cancer, and neurodegenerative diseases. For example, overexpression of PAD4 is associated with the occurrence of rheumatoid arthritis.