LONDON — A sperm donor who unknowingly carried a gene variant that can cause cancers fathered nearly 200 children across Europe, some of whom have already died, according to an investigation published Wednesday by a group of 14 European news organizations.

The man started donating sperm at Denmark’s European Sperm Bank in 2005, which was sold to women from at least a dozen countries over roughly 17 years, the organizations reported. He is healthy and had passed the donor screening checks, but some of his cells had a mutation in a gene called TP53, the healthy version of which helps prevent the body’s cells from turning malignant. Donor checks don’t often look for rare mutations.

Most of the man’s cells do not have the mutation, but up to 20% of the sperm he makes do. Any child born fro

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