Fig. 2. Bts1 is a transcriptional activator in vivo. Expression of collier (col) revealed by in situ hybridization at the head-trunk junction of the Drosophila blastoderm in wild-type embryos (A) and in btd mutant embryos carrying one copy of bts1 under control of btd regulatory elements (B). btd mutants show no expression of col (not shown). bts1 can partially rescue col expression in btd mutants, in a correct spatiotemporal manner.