Ala 9]α-factor, and native α-factor (X = amino acids). Their biological activities (halo, gene induction, and affinity) were measured using S. cerevisiae Y7925 and LM102 and compared with those of native α-factor (100%). G protein-coupled receptor was expressed in strain LM102 containing pESC-LEU-STE2 vector. [Dap 6 ,D-Ala 9 ]α-factor with weak halo activity (10%) showed the highest receptor affinity (> 230%) and the highest gene induction activity (167%). [Arg 6 ,D-Ala 9 ]α-factor showed the highest halo activity (2,000%). The number of active binding sites per cell (about 20,000 for strain LM102) was determined using a newlydesigned fluorescence-based detector, [Arg 6 ,D-Ala 9 ]α-factor-Edan, with high sensitivity (12,500-fold higher than the absorption-based detector [Orn 6 ]α-factor-[Cys]3).