Senior study author Jan Nolta, director of the University of California at Davis stem-cell program, tested the safety of gene transfer into bone marrow stem cells from human donors in more than 600 mice.

The study, published online in Molecular Therapy, found none of the transplanted mice developed leukemia or solid tumors caused by the gene therapy treatment during the evaluation period of up to 18 months.

"These data are critical for advancing stem-cell research leading toward therapies," Nolta said in a statement. "We've shown that adult stem cells follow natural cues to reach target locations, they function normally when they get there and do not exhibit the unchecked cell growth that is the hallmark of cancer."

Gene therapy trials using human bone marrow cells in the early 1990s included roughly 1,000 patients worldwide. In 2000, a leukemia-like condition emerged in three participants in a clinical trial in France, halted the trial.

"Due to the significantly large number of study animals, our investigation further illuminates the safety of gene therapy using hematopoietic stem cells," lead author Gerhard Bauer said.