Substantial heterogeneity exists in the drug responsiveness of individual cells, even in the case of a seemingly homogeneous cell population.176 To understand more fully the manner in which individuals respond to exogenous substances, investigators have sought to develop techniques that can measure specific patterns of gene expression in single cells.
Several methodologies, such as subtractive hybridization, comparative analysis of expressed sequence tags, and differential display, have been used to identify patterns of gene expression. Most of these approaches can be used to assess gene expression in bulk tissue or millions of cells; they cannot analyze expression among transcript populations completely, and they also might not detect transcripts expressed at low levels. Microarrays have also been used to analyze gene expression, but their main strength lies in analyzing transcripts that have been previously identified. On the other hand, the serial analysis of gene ex-
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pression (SAGE) method - 9 does provide rapid, comprehensive, and quantitative analysis of gene expression patterns, and modifications of SAGE permit analysis of gene expression on as few as 500-5000 cells.180 The modifications affect the first few steps of the SAGE procedure, from RNA isolation to PCR, but do not alter the basic principle. Additional modifications of SAGE have now led to techniques capable of exploring gene expression patterns in single cells.181,182
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