The use of embyronic stem cells is highly controversial
US researchers say they can now watch genetic diseases unfolding in the laboratory after finding a way to make large numbers of affected cells.
The advance, reported in the journal Nature, used skin cells from a child with a devastating genetic condition.
These were turned first into stem cells - the body's master cells - and then into nerve cells, which started to die as the gene fault took hold.
The team said the earliest stages of the disease could now be identified.
The child involved suffered from spinal muscular atrophy (SMA), a condition which kills nerve cells controlling muscles, causing paralysis and ultimately death, usually by the age of two.
While doctors know these cells are dying, they have not been able to watch the process in close-up, and look for clues as to exactly how the genetic fault carried by these children is affecting them.
The new technique could in theory provide them with as many nerve cells as they need.
All cells in the body - including skin and nerve cells - originate from the same stem cells found in the early embryo.
Scientists have already found ways to "persuade" skin cells to turn back the clock and become stem cells again.
Once in this state, they can then encourage them to become brand new cells of a different type.
In this case, researchers from the University of Wisconsin-Madison and the University of Missouri-Columbia coaxed them into becoming neurons.
As these nerve cells carried the SMA gene fault, the team observed that, after thriving for a month, they started dying.
Professor Clive Svendsen, one of the researchers, said: "Now you can replay the human disease over and over in the dish and ask what are the very early steps that began the process - it's an incredibly powerful tool."
Another researcher, Dr Allison Ebert, said: "If we start to understand more of the mechanism of why the motor neurons specifically affected in the disease are dying, then potentially new therapies can be developed to intervene at particular times early in development."
In theory, SMA is not the only genetic disease which could be studied using this method - the researchers say disorders such as Huntington's disease could be investigated this way.
Professor Robin Lovell-Badge, of the Medical Research Council's National Institute for Medical Research in London, said that the technique would undoubtedly become "an important tool" for researchers.
He said: "While I would be cautious about interpreting too much from this particular research, and would like to see it repeated using more stem cell lines, I expect this to fairly quickly become a significant aid to drug discovery."