Functional genomics: new insights into the 'function' of low levels of gene expression in stem cells.

Understanding the global gene expression profile of stem cells and their multilineage differentiation will be essential for their ultimate therapeutic application. Efforts to characterize stem cells have relied on analyzing the genome-wide expression profiles that are biased towards the identification of genes that display the most pronounced differential expression. Rather than being viewed as a "blank" state, recent studies suggest that stem cells express low levels of multiple lineage specific genes prior to differentiation, a phenomenon known as "lineage priming." It is not likely that low levels of lineage-specific genes produce sufficient amounts of differentiation factors, but rather to provide rapid transcription to a wide range of lineage programs prior to differentiation. Thus, stem cell differentiation may involve the elimination of other potential pathways and the activation of a specific lineage program.