TALEN

A TALEN is a programmable DNA-cutter that reads DNA almost like beads on a string, one protein piece per letter. It came between zinc finger nucleases and CRISPR, and its appeal was a simpler code: each module reliably recognizes one DNA base, so you assemble modules in order to spell out your target.

TALEN stands for Transcription Activator-Like Effector Nuclease. The DNA-binding part comes from TAL effector proteins, originally from plant-infecting bacteria, built of repeating units where a single pair of amino acids determines which base that unit binds. This array is fused to the FokI cutting domain; as with zinc finger nucleases, two TALENs bind facing each other and together create a double-strand break.

Because the one-module-per-base code is straightforward, TALENs are easier to design than zinc finger nucleases and can be very precise. But each protein is large and must be custom-built for every target, which is slower and costlier than simply swapping a CRISPR guide RNA — so TALENs remain useful in specific cases rather than as the everyday default.