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FIGURE 4 Illustration of the distribution of protein sequences in the genome of Mycoplasma genitalium.SOURCE: Reprinted with permission from Elsevier Science (Teichmann et al., 1999).

half of the protein structures have been determined experimentally, about a third have structures that can be predicted “well” by homology modeling, and about half are completely unknown. This distribution, which is similar in other organisms, provides an illustration of the scale of missing “fold space.” Filling in these missing structures via a number of different strategies is a goal of current genomics research. This information is important not only because it provides fundamental insights into how different protein folds can be used for the same or different functions but also because it is of practical importance in drug design.