It has also been reported that about 40% of spacers from lactic acid bacteria, namely Streptococcus thermophiles, have a homologue corresponding to either phage or plasmid DNA sequences among the isolated and sequenced phage's genome [3]. Thus, the spacers are proposed to be derived from foreign DNA including phage and plasmids. Bacteria generally acquire new spacers once they are exposed to new viruses in order to confer resistance to the cognate virus. Therefore, spacers present in a certain bacterium typically reveal the historical record of phages that the bacterium has been exposed to [4]. The leader sequence is commonly located 5' to the CRISPR locus and is rich in AT nucleotides. However, it has been demonstrated that the leader sequence is incapable of encoding RNAs or proteins, and that it is located next to the short repeats. Apart from the leader sequence, the genes encoding Cas proteins are found to be localized either upstream or downstream of the CRISPR locus.The CRISPR/Cas system is postulated to be formed in three stages. In the adaption stage, bacterial cells acquire sequences derived from foreign DNA such as viral DNA, and then incorporate them into the bacterial CRISPR array. During the expression stage, precursor CRISPR RNA (crRNA)