Using stem cell technology, scientists can now create mammalian cells with double copies of mutant genes, greatly aiding research into genetic diseases, says an article in this week’s issue of the journal Nature.In many genetic diseases, both copies of a gene have to be mutant before the disease is apparent. If just one gene is mutant, its defects are covered by the other healthy gene.Scientists typically use specially bred mice with double mutant genes to study genetic diseases. But intensive breeding programs have been required in order to produce enough mice to keep up with laboratory demand.This new method, developed separately by researchers in the UK and Japan, provides a useful source of mammalian cells that scientists can use to study genes and the biochemical pathways those genes encode.