Human embryonic stem cells hold great promise for studying and treating disease and for the practice of regenerative medicine. However, more must be learned to ensure that the cells that may one day be transplanted into humans are safe.

In one of many safety-related studies going on worldwide, scientists at the UCLA Broad Stem Cell Research Center have discovered that variations in X-chromosome inactivation take place very early in the process of culturing female human embryonic stem cells lines. This finding will play an important role in ensuring the safety of cells grown for therapeutic use and also may have implications in the development of cancer.

Chromosomal and genetic variations found in human embryonic stem cells lines have been attributed in large part to the various culture conditions in which they're grown, said Tamar Dvash, first author of the study and a postdoctoral fellow at UCLA. However, the work by the Broad Stem Cell Research Center scientists reveals that very early in their growth, female human embryonic stem cells already show variation in the inactivation of the X chromosome.

"It suggests that culture conditions and methods in the derivation of human embryonic stem cells could be further improved to achieve a uniform pattern of X-chromosome inactivation," said Guoping Fan, an associate professor of human genetics and senior author of the study.

The study appears June 25 in the early online edition of the journal PLoS ONE, a publication of the Public Library of Science.

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