In stage 12 embryos, sna is expressed in both the anterior and posterior lobes of the optic primordium.

Expression assayed at stages 9, 11, 13, and 17. Expression may be continuous between assayed stages in some tissues.

sna transcript expression is first detected during the cellular blastoderm stage, in presumptive mesodermal cells. Mesodermal expression fades during germband elongation, when sna expression begins to be detected in the ectoderm. The initial pair-rule pattern of sna expression gives way to a segmentally repeated pattern of cells interconnected by longitudinal rows of cells also expressing sna. Neuroblasts will delaminate at approximately the sites of these longitudinal rows of cells during the S1 phase of neuroblast segregation. At about 5.5 hours, sna expression is seen in both the nascent neuroblasts and in ectodermal cells. As neuroblast segregation nears completion at about 5.5 to 6 hours, sna expression is restricted to the neuroblast layer. By 6.5 to 7 hours, sna transcript is expressed in all neuroblasts and in PNS precursors. As in the CNS, sna transcript expression in the PNS begins in neurectodermal patches. Later, sna transcript is detected in sensory mother cells. At about 9 to 10 hours, sna is expressed in the postmitotic cells of the PNS and CNS. Starting at about 11 hours,and continuing into the first larval instar, sna is expressed in the precursors of the wing and haltere discs.

In wild type embryos at embryonic cycle 13, sna transcript is found at low levels ventrally in a region 12 to 14 nuclei-wide. At embryonic cycle 14, sna transcript is expressed at higher levels and in about an 18 cell-wide strip (the presumptive mesoderm), and the borders of expression continue to be fuzzy. By the onset of cellularization, the borders of sna transcript expression in the 18 ventral cells have sharpened, and end at the mesoderm-neurectoderm boundary. In a twi mutant background, level of sna transcript remains low, and is limited to the ventral-most 12-14 cells.

The early accumulation of sna transcript in wild-type embryos is consistent with the role of sna in mesoderm formation, but its later expression pattern suggests that sna might have additional functions. Prior to gastrulation, sna transcript is detected in the mesoderm and anterior midgut anlagen. Before germ band elongation at stages 7-9, sna transcript is present at the dorsal surface of the amnioproctodeal invagination and in the anterior midgut rudiment. Starting at late stage 8, and persisting through stage 11, sna transcript is also observed in some large neurectodermal cells, presumably neuroblast precursors and segregated neuroblasts. Later in embryogenesis, sna is expressed in cells which might be the precursors of the peripheral nervous system (stage 10), P cells (stages 11-14), dorsal mesothoracic disc (stage 13), dorsal metathoracic disc (stage 13), and Bolwig photoreceptor organs (stage 13).

Both twi and sna transcript are first detectable during nuclear cycle 12 as a diffuse band half the width of the presumptive mesoderm. Although the expression patterns of sna and twi transcript and protein are similar early in embryogenesis, there are some subtle differences. At mid-cellularization, the sna transcript and protein expression boundaries sharply delimit the presumptive mesoderm. At the same time, twi transcript and protein is expressed in a gradient that extends slightly past the sna expression zone into the presumptive ectoderm. The twi transcript and protein expression pattern gets sharper later, during gastrulation. twi protein is expressed in the mesoderm throughout germ band extension, whereas the sna protein product disappears from the mesoderm partway through germ band extension, and appears in neurectodermal cells which might be neuroblasts.

High levels of sna transcript are detected in northern blots of embryos 2-4 hours after egg-laying. The levels decrease dramatically, but are still detectable, 4-6 hours after egg-laying.

At stage 5 of embryogenesis, sna protein is expressed ventrally, in the presumptive mesoderm, while esg protein is expressed in the dorsal region. By stage 13, both sna and esg proteins are expressed in presumptive wing and haltere discs. At stage 15, sna and esg proteins are also expressed in the genital disc. The esg protein is detected in the posterior spiracle and in the presumptive leg discs at stage 15, while the sna protein is expressed in a far smaller number of cells in the presumptive leg discs.

While sna transcript is expressed at high levels in in the neurectoderm, sna protein is not detected in these cells. sna transcript and protein expression overlap in other tissues such as presumptive mesodermal cells during the blastoderm stage, and in postmitotic neurons.

sna transcript is first detected during the syncytial blastoderm stage, whereas sna protein expression is first detected during gastrulation. After this initial lag, the expression of sna protein and transcript overlap to a great extent, with some differences apparent after the start of germ band elongation at stage 7. In stage 6 of embryogenesis, sna protein is detected in the mesoderm and the anterior midgut primordium. At stage 7, it is found in the anterior and posterior transverse furrows, as well as the cephalic furrow. Between stages 9 and 11, sna protein is present in the neurectoderm. In late embryogenesis, weak staining is observed in presumptive wing and haltere discs.

Both twi and sna transcript are first detectable during nuclear cycle 12 as a diffuse band half the width of the presumptive mesoderm. Although the expression patterns of sna and twi transcript and protein are similar early in embryogenesis, there are some subtle differences. At mid-cellularization, the sna transcript and protein expression boundaries sharply delimit the presumptive mesoderm. At the same time, twi transcript and protein is expressed in a gradient that extends past the sna expression zone into the presumptive ectoderm. The twi transcript and protein expression pattern gets sharper later, during gastrulation. twi protein is expressed in the mesoderm throughout germ band extension, whereas the sna protein product disappears from the mesoderm partway through germ band extension, and appears in neurectodermal cells which might be neuroblasts.