Tightly regulated doxycycline-inducible GFP expression in ES cells and hematopoietic cells. (A) ES cells were cultured with indicated concentrations of doxycycline and analyzed by FACS for GFP fluorescence after 3 days. Data are mean ± SD of triplicate cultures from a representative experiment. (B) Fetal liver cells were depleted of red cells and cultured in serum-free medium supplemented with cytokines and the indicated concentrations of doxycycline. GFP fluorescence was measured by FACS after 3 days. Data are mean ± SD of duplicate cultures from a representative experiment. (C) Lethally irradiated mice were transplanted with GFP-inducible fetal liver cells. Four months after transplantation, half of the mice were given doxycycline in the drinking water for 1 week and the mice were then killed for bone marrow analysis. Representative FACS profiles depicting GFP expression within the lineage ⁻c-kit⁺Sca1⁺ compartment.