Bioluminescence identifies early stages in tumor development. Serial images of the development of a spontaneous ear tumor in a TAX-LUC animal. Color indicates flux is greater than or equal to 0.3 × 10⁴ photons/s/cm²/sr. (B) Bioluminescence (white arrow) generated by D-luciferin on an otherwise unremarkable ear or tail juxtaposed to corresponding histology. H&E, bar = 100 μm. Inset is TAX IHC on a serial section. (C) Palpable nodules that develop then resolve on the tails of TAX-LUC animals. Histologic sections through the nodules consistently involve inflammatory admixture but presence of malignant LGL cells is variable.