1 November 2012 Last updated at 23:09 ET By James Gallagher Health and science reporter, BBC News

Boys left infertile by childhood cancer treatment may one day be able to produce healthy sperm by using stored stem cells, monkey research suggests.

Chemotherapy and radiotherapy can kill tumours and the cells which make sperm.

A study, published in the journal Cell Stem Cell, extracted sperm-producing stem cells before cancer treatment and later placed them back into the monkey.

Sperm which could fertilise an egg were produced, which experts labelled a "milestone" in research.

Most men who have cancer treatment which could affect their fertility can choose to freeze sperm before their treatment starts. This is not an option for patients who have not yet gone through puberty.

These are issues we still must work through, but this study does show us the concept is feasible

However, they do have the spermatogonial stem cells which would start to produce sperm in their teenage years.

The researchers at the University of Pittsburgh and the Magee-Womens Research Institute took samples of the stem cells from macaques and stored them in a freezer.

The monkeys were then given a chemotherapy drug.

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Stem cell op may 'restore sperm'