Fig. 4. Transcripts arising from the scs promoter in the BX-C. Organization of the genomic DNA around Fab-7 in Fab-7² (left), _promscs (middle) and scs_prom (right) flies. The promoter active in _promscs and scs_prom is drawn. The A and B strand-specific probes are shown in red: probe A, bases 84,732 to 86,947 in the BX-C sequence (Martin et al., 1995); probe B, 82,554-84,732. The expression pattern of the transcripts in wild type (or Fab-7²), _promscs and scs_prom embryos at 4 and 12 hours of development are shown below. Parasegments are indicated. In scs_prom, we detected a similar expression pattern with a probe spanning the iab-8PRE (not shown; from 59,446 to 62,117). It should be noticed that the intensity of rightwards transcription is higher from scs_prom than leftwards transcription originating from _promscs. Although the signal usually appears after 1 hour of incubation with the alkaline substrates, with probe B, we had to wait more than twice as long as we did with probe A.