Figure 2.

(A) The retroviral ecdysone-inducible system yields virtually 100% inducible clones. Depiction of one of five candidate U87MG-inducible host clones, U87.23, infected with pI-LacZ and selected for hygromycin-resistant colonies. Each plate contained ≈200 colonies. The bottom dish was induced with 0.5 μM muristerone A for 24 h, fixed, and stained for β-galactosidase protein. The top dish was left uninduced, fixed, and also stained. Efficiency of LacZ expression was comparable for the remaining four clonal hosts (data not shown). (B) Complete recruitment by inducer at the single-cell level. The same U87.23 LacZ-inducible cell line was plated on coverslips and induced with 0.5 mM muristerone A. Cells were fixed and stained for β-galactosidase after 8 h (Lower Left), 12 h (Lower Right) and 24 h (Upper Right), or left uninduced (Upper Left). Note that even as early as 8 h all cells are recruited to express the inducible transgene.