slou transcripts are strongly expressed between 8 and 24 hours of embryogenesis and at a lower level in adult females. Much lower levels are detected in larvae, pupae, and adult males.

Very low levels of early developmental slou expression were detected.

slou is expressed in muscle progenitors that lead to lateral oblique muscle 1 and ventral transverse muscle 1 but persists only in the latter. It is expressed in progenitors for ventral acute muscles 1, 2, and 3 and in ventral adult muscle precursor cells. It is also expressed in the dorsal transverse muscle progenitors and perhaps dorsal oblique muscle 3.

slou expression is first detected in a single cell in the ventral mesoderm of each hemisegment which divides to produce a pair of slou-expressing progeny. At the end of stage 11 a second cluster of expressing cells appears in the ventral mesoderm consisting of 4 cells/segment in the abdominal segments and more in the thoracic segments. As the germband retracts, another more pair of more mesodermal cells begins to express slou in the abdominal segments. These early patterns are a prelude to muscle formation and the recruitment of additonal slou-expressing nuclei as myoblasts fuse with slou-expressing muscle founders during and after germband retraction. Finally, slou is expressed in three groups of muscles in each abdominal hemisegment, ventral transverse muscle 1, ventral acute muscle 2 and dorsal transverse muscle with special patterns in the thorax and more posteriorly. slou expression is observed shortly after mesodermal expression begins and develops into a segmentally repeated pattern after germband retraction. Expression begins in the posterior midgut after fusion in stage 12-13 hour embryos.

slou protein is expressed in small subsets of cells in the somatic mesoderm and in a small number of muscles. It is initially expressed at stage 11 in a single mesodermal cell located ventrally in each hemisegment. As embryonic development proceeds, it is expressed in the progeny of this cell as well as in two other clusters of mesodermal cells. The pattern of staining is different between thoracic and abdominal segments and is described in detail. In stage 13, slou-expressing cells fuse with each other and with other cells to form syncytia which continue to express slou protein. In late embryonic stages, slou protein is expressed in three muscle fibers in each abdominal hemisegment which have been identified as muscles 25, 27, and 18. Again the pattern is slightly different in the thoracic and gnathal segments than in the abdominal segments. slou protein is also expressed in cells of the midgut. Expression is confined to a narrow band of cells directly anterior to the first constriction that will form at this position. Later, all cells in the region of the loop derived from this region of the midgut contain slou protein. slou expression in the CNS starts shortly after mesodermal expression. In early stage 11, a single neuronal precursor which is thought to be a ganglion mother cell is stained in each segment. During stages 11 and 12, a cluster of cells is stained at this position, and a separate cluster of ganglion mother cells gradually appears. After germ band retraction, these cells assume a V-shaped pattern. Eventually, the slou-expressing neurons become located lateral to the axonal fiber tracts. slou protein is also expressed in distinct cells of the supraoesophageal ganglion.