Tom is strongly expressed in neuroepithelial cells in the optic lobe and downregulated in neuroblasts.

Tom transcripts are detected in third instar larval eye discs in the morphogenetic furrow in stripes just anterior and posterior to the band of dpp expression in the furrow. They are detected in the pupal wing at 8h APF in large clusters of proximal campaniform sensilla.

Tom transcripts appear in 2-4hr embryos, peak in 4-8 and 8-12hr embryos and then decline. They are barely detectable after the first instar larval stage. Tom transcripts are detected in the anteriormost and central regions of early blastoderm embryos. In late blastoderm embryos, Tom is uniformly expressed except in the ventralmost regions where its expression is repressed by sna. From gastrulation until the end of germ band elongation, Tom is expressed throughout the mesoderm though by stage 11, expression occurs in a segmental pattern and is observed in tracheal pit precursors. Tom is also expressed in the ectoderm. During germband elongation and neuroblast segregation, Tom transcripts are excluded from one or two rows of ectodermal and mesectodermal cells in each segment. Later, Tom transcripts are transiently absent from segmental, bilaterally symmetric patches of cells. At the beginning of germ band retraction, Tom is more weakly expressed in the ventral region than in the dorsal region of the ectoderm. Expression rapidly decays during germ band retraction. After dorsal closure, Tom expression is observed only in lymph glands. In eye discs, Tom is expressed anterior and posterior to the morphogenetic furrow.