Proportion of tags mapping to the murine genome. In order to search for probable murine contaminants among our set of tags, we mapped to the murine genome all tags that do not match the human genome sequence and for which we could not find any other human origin. This figure shows the percentage of these tags that could be localized on the murine nuclear and mitochondrial genome, for each public LongSAGE library. The identification number of the LongSAGE libraries in the Gene Expression Omnibus repository is indicated on the x axis. All embryonic stem cells were propagated on murine embryonic fibroblasts excepted the ones used to construct the library indicated by an arrow.