There are 4 JAK proteins: JAK1, JAK2, JAK3 and TYK2.[1] JAKs contains a FERM domain (approximately 400 residues), an SH2-related domain (approximately 100 residues), a kinase domain (approximately 250 residues) and a pseudokinase domain (approximately 300 residues).[2] The kinase domain is vital for JAK activity, since it allows JAKs to phosphorylate (add phosphate groups to) proteins.