Tissues and organs are composed of various kinds of cells including stem cells, progenitors, and terminally differentiated cells that have large variations of gene expression. Within a seemingly homogeneous cell population, gene expression levels may differ dramatically on a cell-to-cell level. However, differences or variations of individual cells are masked by the averaging effect of pooled samples in population analysis. Therefore, analyzing transcript levels for multiple genes across multiple individual cells could be key for understanding the unique characteristics of individual cells and for clarifying the complicated mechanisms controlling the function of individual cells. Recent advances in cDNA amplification techniques from single cell samples allow for analyzing gene expression patterns in individual cells. Amplified cDNAs are applicable to measurements of gene expression systems such as microarrays, DNA sequencing, and TaqMan assay-based high throughput nanofluidic real time PCR array analysis. Gene expression profiling at the single cell level allows us to identify specific sub-populations in heterogeneous cell populations in tissues and clarify the complex networks controlling the function of individual cells.
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