Staining of 293FT cells cotransfected with the Env expression vector and pKcTac-Halo in the presence or absence of soluble CD4 by membrane-impermeable ligand, Alexa Fluor 488 (AF488). Soluble CD4 (0.1 μM) was used to induce the conformational changes of gp120. The names of the expression vectors used were shown on top. Merge: merged images of bright field and AF488 signals. The bar indicates 40 μm.