Abstract: A computer implemented method for characterizing one or more sequences by generating index values representing portions of the sequences and finding characterizing index values based on a comparison of the index values. The index values may be obtained by applying one or more mask over each sequence. The modified masks may have associated weightings and index values obtained using modified masks may be retained in the index only if the weightings are above a threshold value. Characterising index values may also be assessed for for their degree of uniqueness. Characterizing indexes may be used for predicting correlation between a sample sequence and one or more reference sequences. Biological monitoring systems utilising the characterizing index values are also disclosed. A biological indicator may be generatgenerated using one or more characterizing index values obtained by the above method and be used to produce an indicator that undergoes a property change in the presence of the one or more sequence.