The article by Brouns et al. (2008) elucidates the molecular mechanism by which prokaryotes use CRISPR (clusters of regularly interspaced short palindromic repeats) to defend against viral infections. The authors focus on the *Escherichia coli* K12 CRISPR/cas system, which consists of eight *cas* genes. They identify a protein complex called Cascade, composed of five Cas proteins (CasA, CasB, CasC, CasD, and CasE), that is essential for antiviral defense. Cascade cleaves a pre-crRNA precursor into mature crRNAs, which serve as guide RNAs to target and interfere with viral nucleic acids. The study demonstrates that the formation of mature crRNAs by the Cascade complex is crucial for the antiviral response. Additionally, the authors show that the crRNAs produced by Cascade are capable of reducing the sensitivity of *E. coli* to phage Lambda, confirming the role of Cascade in mediating this defense mechanism. The findings provide insights into the adaptive and inheritable defense system in prokaryotes, highlighting the importance of CRISPR-based defense in protecting against viral infections.The article by Brouns et al. (2008) elucidates the molecular mechanism by which prokaryotes use CRISPR (clusters of regularly interspaced short palindromic repeats) to defend against viral infections. The authors focus on the *Escherichia coli* K12 CRISPR/cas system, which consists of eight *cas* genes. They identify a protein complex called Cascade, composed of five Cas proteins (CasA, CasB, CasC, CasD, and CasE), that is essential for antiviral defense. Cascade cleaves a pre-crRNA precursor into mature crRNAs, which serve as guide RNAs to target and interfere with viral nucleic acids. The study demonstrates that the formation of mature crRNAs by the Cascade complex is crucial for the antiviral response. Additionally, the authors show that the crRNAs produced by Cascade are capable of reducing the sensitivity of *E. coli* to phage Lambda, confirming the role of Cascade in mediating this defense mechanism. The findings provide insights into the adaptive and inheritable defense system in prokaryotes, highlighting the importance of CRISPR-based defense in protecting against viral infections.