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Figure 1 | The formation of a micronucleus.

However, if any chromosomes lag behind the rest and are still in the middle of the dividing cell during telophase, they become isolated and can form a separate small nucleus called a micronucleus.

Deficiencies in the formation of nuclear-pore complexes in the nuclear envelope that surrounds a micronucleus might hinder the entry of the proteins needed for successful repair of DNA damage and for stabilization of the nuclear envelope, which might explain why micronuclei are prone to rupture7 and chromothripsis3,4.

Writing in Nature, Liu et al.5 report that the nuclear-envelope structure that encloses a micronucleus lacks some of the components of the nuclear envelope that surrounds a nucleus.

When nuclear-envelope material reassembles around a lagging chromosome, the result is a rupture-prone structure termed a micronucleus that can undergo large-scale DNA damage and is linked to cancer2–4.