Fluorescence microscopy of cells expressing FRB–F[1,2] and FKBP–F[3]. (A) CHO DUKX-B11 cells stably expressing the fusions were incubated with fMTX at a final concentration of 10 μM, with or without addition of 20 nM rapamycin, for 22 h at 37°C. (B) COS-7 cells were transiently transfected with the fusions and treated and analyzed by fluorescence microscopy as in A. Controls include the positive interaction by leucine zipper formation (ZIP–F[1,2] + ZIP–F[3]) and negative controls for noninteracting pairs (FRB–F[1,2] + ZIP–F[3]; ZIP–F[1,2] + FKBP–F[3], not shown).