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Colocalization of cyclin T and Pol IIa on Drosophila polytene chromosomes. Shown here is an antibody staining of cyclin T (green) and the hypophosphorylated form of RNA polymerase II, Pol IIa (red) on polytene chromosomes from non-heat-shocked larvae. P-TEFb, a heterodimer of cyclin T and Cdk9, colocalizes to sites of active transcription elongation sites, supporting a model in which P-TEFb acts to stimulate promoter-paused Pol II to enter into productive elongation. (For details, see Lis et al., p. 792.)