  Transcription factor combinations determine gene locus activity and thereby
cell identity. However, the precise link between concentrations of such
activating transcription factors and target-gene activity is ambiguous. Here we
investigate this link for the gap gene dependent activation of the even-skipped
(eve) locus in the Drosophila embryo. We simultaneously measure the
spatiotemporal gap gene concentrations in hemizygous and homozygous gap
mutants, and link these to eve activity. Although changes in expression extend
well beyond the genetically manipulated gene, nearly all expression
alternations approximate the canonical combinations of activating levels in
wild-type, sometimes necessitating pattern shifts. Expression levels that
diverge from the wild-type repertoire still drive locus activation. Specific
stripes in the homozygous mutants show partial penetrance, justifying their
renown variable phenotypes. However, all eve stripes appear at highly
reproducible positions, even though a broader span of gap gene expression
levels activates eve. Our results suggest a correction capacity of the gap gene
network and set constraints on the activity of multi-enhancer gene loci.
