Expression of the ferrochelatase promoter transgene during primary in vitro differentiation.

ES cells containing the −4.0 kb transgene were differentiated in a liquid culture (primary differentiation) for 5 days. Original magnification, × 100. (A) Bright-field images and (B) green fluorescent images were taken from day 0 (undifferentiated) through day 5 using identical exposure times. A transgene-negative, 4-day differentiated EB was included as a negative control (TG-D4, A and B). Arrows point to fluorescing cell clusters. (C) Quantitation of green fluorescence was determined using flow cytometry. Each histogram represents the fluorescent intensities of approximately 50 000 cells determined at each 24-hour time point. Relative EGFP intensities are indicated above each histogram.