such carriers appear to be uniformly distributed around the conducting cells. Inside the cell, IAAH dissociates at the higher pH of the cytoplasm into IAA- and H+ and can exit (efflux) the cell only through the activity of the auxin efflux car¬riers. These carriers, which are precisely aligned with the direc¬tion of auxin transport, are known as PIN proteins. They are named after the pin-shaped inflorescences formed by the pinl mutant of Arabidopsis. PIN proteins are constantly being cycled