Fig. 2. Activity of the first intron in GFP reporter gene expression. (A) Construction of reporter genes. Zebrafish gata1 locus is shown at the top, where open and striped bars denote the upstream region and the first intron, respectively. (B) GFP expression of the 8.1kG1-eGFP and 8.1kG1dI1-eGFP constructs. Top panels indicate the endogenous gata1 expression analyzed by whole mount in situ hybridization. ICM, intermediate cell mass. (C) Region for counting GFP-expressing cells. (D) Ratios of GFP-expressing cell numbers between the 8.1kG1-eGFP- and 8.1kG1dI1-eGFP-injected embryos. Embryos that did express GFP in the ICM at 24 hours were selected and further analyzed at the embryonic days indicated. Concentration of the injected DNA was 10, 25 or 50 µg/ml, as indicating by white circles, black circles and white squares, respectively. Five pictures of each embryo were taken at a 30 msecond video rate, and numbers of cells expressing GFP in the square shown in C were counted and averaged. Percentages were calculated from the mean of positive cell numbers in examined embryos (n=21 and n=37 for 8.1kG1-eGFP and 8.1kG1dI1-eGFP, respectively).