Poxm is first detected in the somatic mesoderm at embryonic stage 10. During stage 10, it is expressed in segmentally repeated mesodermal stripes underlying the ectodermal parasegments 2-14, in the head mesoderm, the proctodeal anlage, and in a group of ectodermal cells from the clypeolabrum, which are thought to correspond to part of the esophagus anlage. Poxm is expressed in the high twi domain in the lateral and ventral mesoderm. Its expression is repressed in the dorsal portion of each segment by dpp. The number of Poxm-expressing cells increases with distance from the dorsal margin, forming a triangular pattern. Its expression domain is complementary to that of tin. During stage 11, Poxm expression is restricted to fewer cells, some of which will form subsets of muscle progenitors and cells of the promuscular clusters. During germband retraction, Poxm disappears from the most anterior mesodermal stripe and the telson. By stage 12, Poxm expression is maintained in only six cells in each of the abdominal segments A1-7. These are the founder cells of DO3, DT1, VA1-3, and the ventral adult muscle precursor. At this time more cells express Poxm in the thoracic segments than in the abdominal segments. As myoblast fusion proceeds during stage 13, the number of Poxm nuclei increases as it is expressed in the precursors of muscles DT1 and VA1-3. During stage 14, Poxm expression begins to be reduced in ventral clusters and in the dorsolater regions from which it disappears during stage 16 and is gone by stage 17. Outside of the mesoderm, high levels of Poxm are observed in the esophagus and hindgut throughout embryogenesis.

Poxm protein is expressed in the somatopleura, giving rise to the somatic musculature.