Imp transcripts are detected early and late in embryogenesis but transcripts containing the 3' UTR extension (~8.5 kb) are only detected at later stages and appear after the maternal-to-zygotic transition. The 3' UTR extension isoforms are highly enriched in nervous system tissues.

Imp expression at embryonic stages 1-4 is localized to the posterior pole of the embryo.

Immunolocalization shows that Imp is expressed in the -- and pole cells in blastoderm embryos and in the ventral nerve cord in stage 14-16 embryos. Double staining with an antibody to futsch reveals that Imp is expressed in peripheral sensory neurons and some futsch-negative cells surrounding them. Overlap with pros expression reveals Imp expression in the scolopale cells of chordotonal organs and in sheath cells of the external sensory organs. No staining was observed in peripheral motor axons or in muscle.

Imp protein is cytoplasmic and present in all cells of the oocyte, follicle cells, nurse cells, and the oocyte. Within the germline, the pattern changes with time. It is initially uniform in the dividing germ line cells of each cyst but becomes rapidly concentrated in the oocyte. By stage S7, the uniform levels in the oocyte are noticably reduced but by stage S8 and S9 Imp levels are elevated in a narrow zone between the nucleus and the anterior and dorsolateral margins of the oocyte (an area which coincides with the region where grk mRNA and protein accumulate). This localizaton is lost by mid-stage 9. Imp also localizes to a crescent at the posterior pole of the oocyte.

Imp protein is found in the oocyte and in nurse cells early in oogenesis but penetration of the antibody into the oocyte is blocked after stage S4.