A EcollacZ construct driven by 2.8 kb ofsna upstream sequence is expressed in a pattern indistinguishable fromwild type sna transcript expression in the CNS, PNS and mesoderm.

2.8 or 6.0 kb of snaupstream region drives EcollacZ expression in a pattern very similar to that of thewild-type sna transcript. The levels of sna promoter-driven EcollacZ transcript are lowerfor the 2.8 kb region as compared to the 6.0 kb upstream region. Ventralexpression detected in nuclear cycle 13 embryos sharpens and becomes limitedto the presumptive mesoderm by cycle 14. Staining extends to the poles,and persists there through gastrulation and germ band extension, unlike endogenoussna transcript, which disappears from the poles by gastrulation. EcollacZexpression is subsequently detected during neurogenesis, first in theneuroblasts of the central nervous system, and later in neurons ofthe peripheral nervous system.