Th, also known as tyrosine 3-monooxygenase, is a rate-limiting enzyme in catecholamine synthesis. It uses tetrahydrobiopterin and molecular oxygen to convert tyrosine to DOPA. It regulates dopamine (DA) neurotransmission at the biosynthesis and reuptake steps. It plays an important role in the physiology of adrenergic neurons. It effects overexpression in lymphocytes on the differentiation and function of T helper cells. Recombinant mouse Th, fused to His-tag at C-terminus, was expressed in insect cell and purified by using conventional chromatography techniques.