Scientists have solved a long-standing mystery: how immune cells aim a dangerous DNA-mutating enzyme precisely at antibody genes without wrecking the rest of the genome.

The study, published in Nature on September 30, 2026 by Noé Seija and colleagues led by Javier M. Di Noia at the Institut de Recherches Cliniques de Montréal, identified two proteins — MLLT1 and MLLT3 — as the gatekeepers. These "histone readers" concentrate the enzyme AID at the few genomic regions where mutations sharpen antibodies.

When the team deleted both proteins from mouse B cells, every AID-dependent mutagenic process ground to a halt. Class-switch recombination and somatic hypermutation vanished entirely — as if AID itself had been deleted, even though it was still present.

The finding also opens a cancer angle. Molecules that inhibit MLLT1 and MLLT3 reduced cancer-linked mutations in lab models, and MLLT1 inhibitors are already in clinical development for certain leukemias.

The discovery matters because it turns a dangerous enzyme into a controllable target — better vaccines on one side, new cancer strategies on the other.