Fig. 3. Multiphoton microscopy imaging of a polytene nucleus. Multiphoton microscopy imaging of a polytene nucleus resolves individual, native, genetic loci and enables transcription factors involved in gene activation to be watched in real time. In the two 3D reconstruction images of a cell nucleus from Drosophila salivary gland tissues shown, heat-shock factor (green) resides in the nucleoplasm at room temperature (22°C, left). Upon heat shock (36.5°C, right), it rapidly associates with chromosomes (red) to activate the transcription of heat-shock genes. Image courtesy of John Lis.