Genome projects are continuously revealing novel genes and the function of many of the genes discovered previously is unknown. The clarification of the function of these genes will be a great challenge and will require coordinated work by many different disciplines. Because many genes share a relatively high homology in different organisms, simple model organisms have been exploited in the analysis of gene function. For example, projects are underway to mutate every single gene in baker's yeast, Saccharomyces cerevisiae (Winzeler et al., 1999). One of the problems that has hampered functional analysis is that simple organisms display only a limited number of observable phenotypes and many mutations do not produce any phenotype at all.