Simple explanation
Before CRISPR, targeting a specific piece of DNA meant building a protein by hand for that exact sequence. TALENs are made of repeating blocks, each one recognising a single DNA letter, so you string the blocks together in the order of your target. It works well — it is just far slower and more expensive than writing a short RNA guide, which is why CRISPR took over so completely.
Go deeper
Transcription activator-like effector nucleases fuse a DNA-binding domain derived from Xanthomonas TAL effectors to the FokI nuclease domain. Each TALE repeat recognises one base pair through a repeat-variable di-residue, giving straightforward one-to-one design. FokI must dimerise to cut, so TALENs are used in pairs flanking the target — a requirement that in itself improves specificity. TALENs have no PAM constraint and access methylated and mitochondrial DNA that Cas9 cannot.
Why they still matter
Three reasons. There is no PAM requirement, so any sequence can be targeted. The obligate dimer design gives an intrinsic specificity advantage. And TALE domains, unlike guide RNAs, work on mitochondrial DNA — no known mechanism imports guide RNA into mitochondria, so base editors built on TALE scaffolds (DdCBEs) are currently the route to editing the mitochondrial genome.
Clinically, TALEN-edited allogeneic CAR-T cells reached patients before any CRISPR therapy did. The 2015 treatment of an infant, Layla Richards, with TALEN-edited donor T cells is one of the field's landmark cases.
Sources
- Nature Biotechnology (Miller et al.) · 2011
A TALE nuclease architecture for efficient genome editing ↗ - Nature (Mok et al.) · 2020
Molecular evolution of a bacterial cytidine deaminase toxin enables mitochondrial base editing ↗