Fig. 1. All three Gli genes are expressed in the ventral somite. Gene expression within the PSM and early somites was examined by radioactive section in situ hybridization on sagittal (A-C') and transverse (D-F') sections through E9.5 embryos. Bright-field images (A-F) are shown with corresponding dark field images (A'-F'). Radioactive section in situ hybridization signal corresponds to silver granules highlighted by dark-field illumination. Dotted lines indicate the boundary between the PSM and first fully formed somite (A-C'). Gli1 is expressed upon somite formation (A,A') in the sclerotome (D,D'). Gli2 is expressed in the anterior PSM and the somites (B,B',E,E'). Gli3 is expressed in the PSM (C,C') and the somites (F,F'). nt, neural tube; dm, dermomyotome; scl, sclerotome.