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What is meant by gene expression? In a multicellular organism, usually every cell contains the complete genome. All genes are present in every nucleus. However, in any given cell, only some of the genes are actually producing mRNA. Many of the genes are inactive and never do anything. Other genes only produce mRNA at certain points in the life of a cell or in response to certain conditions. The production of mRNA is known as gene expression. Genes that do not produce mRNA and do not contribute to the phenotype are not being expressed.