Figure 1.

A photo-mosaic showing Nomarski images of 28 strains of S. cerevisiae containing different tetracycline-regulatable promoter-replacement alleles. Expression of essential genes is repressed by treatment with the tetracycline analog, doxycycline. Armaity Davierwala and colleagues have constructed such alleles for more than 60% of all essential genes and have identified a number of strains with morphological defects - visible as deviations from the typical budding shapes of wild-type cells shown in the dark-shaded region of the montage. Image courtesy of Tim Hughes and Jennifer Haynes, University of Toronto, Canada.