Fig. 6. The pair-rule to segment-polarity transition. The contrasting Runt-dependent activation and repression of slp1 in different cells within the pre-segmental region of the blastoderm embryo is fully explained by the overlapping expression of Ftz, as indicated in this diagram. Also depicted are the expression domains of the pair-rule transcription factors Odd and Prd, which overlap the slp1stripes (Morrissey et al., 1991; Mullen and DiNardo, 1995). Combinatorial regulation by Eve, Runt, Opa and Ftz accounts for all aspects of slp1 regulation, except for activation in even-numbered parasegments. The minimal spatial domain of activity of a Factor X that is proposed to be responsible for this aspect of slp1 expression is depicted in pale blue. Factor X-dependent activation may also contribute to the expanded slp1 expression obtained by transient elimination of eve or run. The possibility that Factor X is active in other cells within the pre-segmental region is indicated by the broken blue line. The strip of cells along the anteroposterior axis drawn at the bottom of this diagram shows the relationship of wg and en expression in each parasegment to that of slp1 and the pair-rule transcription factors.