During development, cells become different from one another because they synthesize and accumulate different proteins. Most of these differences come from changes in gene expression. Specialized cell types result from different genes being turned on or off in a coordinated manner. When a cell becomes a specific cell type, it continues in this role through many subsequent cell generations. This implies that cells remember the changes in gene expression involved in the choice of cell type. How is this achieved? One way is through the euchromatin-heterochromatin conversion, which can be faithfully inherited through cell division. Another way is for an important regulatory protein to activate its own expression as well as the expression of other genes. This gene, once expressed, will maintain its own expression.
As described above, it is usually a combination of gene regulatory proteins, rather than a single protein, that determines when and where gene expression occurs. Certain proteins can be more important than others, though. If all the other factors required for expression of a group of genes are present, a single gene regulatory protein can switch a cell from one developmental pathway to another.
For example, forced expression of the MyoD protein in fibroblasts will cause these cells to form into muscle fibers. In an extreme example, homeotic fibroblasts undifferentiated cells that normally give rise to connective tissue cells
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