The sequence signature is defined as the frequencies of the whole set of short oligonucleotides observed in a sequence (26, 31) . It can be easily obtained thanks to a very fast algorithm derived from the Chaos Game Representation (CGR) (33) , which allows coping with a 1 Mb sequence in a few seconds on a laptop computer. Signatures may be visualized as square images where the color (or gray level) of each pixel represents the frequency of a given oligonucleotide (called word thereafter) (31) (for examples of signatures, see Supplementary Materials 2, 4 and 6).