For illustration purposes, local signatures are developed as vertical vectors and stacked together in genome order to give an overall picture of word usage variations along each genome. In such plots, horizontal lines show the variation in frequency of words along the genome, whereas local changes in word usage appear as vertical breaks ( Figure 1 ). Figure 1 . Signatures (4-letter words and 5 kb windows) along genome for Clostridium acetobutylicum, Deinococcus radiodurans and Mycobacterium tuberculosis. In this kind of displays, lines represent the frequency of words along genome, columns represent signature of windows.