The CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR-associated proteins) adaptive immune systems are discovered in many bacteria and most archaea.These defence systems are encoded by cas (CRISPR-associated) operons that have an extremely diverse architecture.The most crucial step in depiction of cas operons composition is the identification of cas genes or Cas proteins.Furthermore,given the continuous increase of the newly sequenced archaeal and bacterial genomes,the recognition of new Cas proteins is becoming possible,which not only provides candidates for novel.genome editing tools but also helps to understand the prokaryotic immune system better.