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Regulation of pigmentation in the transgenic mouse using elements derived from the lac operon of E. coli. Shown here are the phenotypes of genetically identical mice that carry both a lac repressor transgene and a tyrosinase transgene in which the transcription start site is flanked by three lac operators embedded directly in the mammalian promoter. In the absence of the lactose analog IPTG in the drinking water, the mouse is albino, as shown in the animal on the left. In the presence of IPTG, the lac repressor loses its affinity for the operators, relieving the block to transcription, and the mouse is pigmented. (For details, see Cronin et al., p. 1506.)