Not all stem cells are created equal, a string of new studies suggests: adult cells that are reprogrammed into stem cells carry chemical remnants of the tissue from which they originate, making them distinct from embryonic stem cells. These differences may have important implications for studying fragile X syndrome and other diseases that arise from epigenetic glitches.Embryonic stem (ES) cells can be derived from human embryos, and have the potential to give rise to any type of cell in the body. Induced pluripotent stem (iPS) cells are, in contrast, created in the lab by placing adult cells — typically skin, muscle or blood — in a soup of chemicals, and reversing them into an undifferentiated state.When a Japanese team first debuted iPS cells in 2006, many researchers became hopeful that the cells could be used to model diseases just as well as their polemical counterparts, ES cells. Several groups are using iPS cell models to screen drugs for various diseases.Two reports published on 19 July, however, found that iPS cells in mice are not in a fully reset state. Rather, these reprogrammed cells retain epigenetic markers — chemical add-ons to DNA — of the differentiated tissue from which they originate.This suggests that ES cells might make the best models for disease genes influenced by epigenetic tweaks, such as DNA methylation, a mechanism by which methyl groups attach to DNA and effectively shut off a gene.Read more at...SFARI, August 2010.