A structure named for this letter is used by retinol-binding protein to carry retinol and is used by porins to assist diffusion across the cell membrane. This letter partly names fragments of the membrane receptor APP (10[1])that accumulate in gray matter. Adrenergic receptors denoted by this letter (10[1])are antagonized (10[1])by medications that treat abnormal heart (-5[1])rhythms (10[1])by suppressing (10[1])the fight-or-flight response. The center of green fluorescent protein contains a ■END■