Washington ? Human embryonic stem cells, treasured by researchers because of their potential to help rejuvenate ailing organs, do not remain as ageless and unblemished as scientists once thought, according to a new research report.

Like ordinary cells, stem cells accumulate significant numbers of mutations over time, including several that could cause them to become tumors.

The findings, reported by an international team of scientists Sunday, could bolster those who have been calling upon President Bush to allow the use of federal money to create fresh colonies.

Researchers hope to harvest batches of the cells periodically from master colonies and turn them into various kinds of tissues for transplantation into patients.

But the longer stem cells are cultivated – and the more cell divisions they undergo – the more mutations build up in their genes, Aravinda Chakravarti of Johns Hopkins Medical Institutions and his colleagues reported in Sunday’s issue of the journal Nature Genetics.

The team measured the number of mutations and other DNA abnormalities in nine colonies of cells approved for use by federally funded researchers and compared the extent of the abnormalities before and after each lineage was subjected to a dozen or more rounds of cell division.