Zebra patterns in fly embryos: activation of stripes or repression of interstripes?

Like Gould's zebra, the striped embryo can be viewed as a pattern of both interstripe repression and stripe activation. A growing body of evidence suggests that both processes are at work in regulating pair-rule gene expression. Certain genes, such as ftz, are largely negatively regulated in the interstripes through proximal upstream elements by the striped expression of other pair-rule genes, while others, such as hairy and eve, are largely regulated through distal upstream elements by the aperiodic gap genes (Figure 7). Different gap proteins control different subsets of stripes and interstripes. Combinations of gap proteins regulate transcription by binding to far upstream elements that contain different numbers and types of binding sites.