Fig. 2. Comparative genomics reveals locations of candidate cis-regulatory elements. (A) Sequence comparison of mouse with human, opossum, chicken and zebrafish Tbx4. Colored bars (top) denote location of cis-regulatory intervals; blue boxes indicate position of Tbx2 and Tbx4 exons. All regions with >70% identity over a 100 bp window are colored: blue, exons; red, conserved intergenic sequence; pink, conserved intronic sequences). Block-shaded portions of plots indicate positions of confirmed enhancers (see B): yellow, hindlimb enhancer A (HLEA); green, lung enhancer; pink, hindlimb enhancer B (HLEB); gray, umbilical cord. (B) Genomic regions assayed for enhancer activity. Black bars represent fragments that did not drive consistent expression patterns at E12.5. (C-G) Whole-mount lacZ staining of E12.5 transgenic mouse embryos illustrating expression patterns seen with the different constructs. tr, trachea; lu, lung; uc, umbilical cord.