The final aim of the challenge was to elucidate the mechanisms of epitope reactivity with human antibodies; for this reason, a ''bonus round'' was proposed beside this main challenge. To this end, we have developed a method based on a clustering approach. We grouped the peptides of the training set in about 7000 clusters by using as distance the score of the local alignment. Then, we selected 23 positive, 27 negative and only 4 uncertain clusters by a-posteriori taking into account of the class of the peptides. It is worthwhile mentioning that the small number of negative and positive clusters demostrates that there are many rules underlying peptide reactivity. Each rule has thus a small support; this is probably related to the wide variability of the antibodies.