The on–off switch of CRISPR immunity against phages in Escherichia coli

CRISPR/CAS is a novel prokaryotic defence system against cell invaders. A typical CRISPR/CAS system consists of one or more clusters of regularly interspaced short, often palindromic, repeats (CRISPR) followed at one side by an adenine- and thymine-rich (AT-rich) leader sequence, and a variable set of CAS (CRISPR associated) genes