Carrier proteins transport important polar molecules across the cell membrane. The function of a carrier protein is to ‘smuggle’ important polar molecules across the cell membrane. Carrier proteins are also important in transporting important neurotransmitters such as norepinephrine and dopamine back into the nerves from which they were released. Cocaine acts by inhibiting the carrier proteins for norepinephrine in the peripheral nervous system as well as the carrier proteins for dopamine in the central nervous system. Amphetamine is transported into nerves by the carrier protein for norepinephrine. The uracil portion of the drug is recognized by the transport protein, while the mustard portion is an alkylating agent which reacts with deoxyribonucleic acid (DNA) and prevents DNA from functioning. Some drugs are sufficiently similar in structure to the natural guest that they are accepted by carrier proteins.