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The human genome contains perhaps more than 50,000 genes. A typical cell in the human body is believed to express from 15,000 to 25,000 proteins. The majority of these proteins are expressed in all the cells, and are known as housekeeping genes. These are proteins involved in vital cell functions, including production of energy, oxidation, biosynthesis, regulation of cell cycle, and cell growth. The rest of the proteins are expressed differentially in various cell types. For example, some genes are expressed only in liver cells, muscle cells, or neurons, and not in any other cell types. These differentially expressed genes define the phenotype and function of a particular cell type. The identification of genes solely expressed in one cell type may shed light on mechanisms of cellular function and gene interaction. [ABSTRACT FROM AUTHOR]