Examples myode

Most cancer cells show abnormal differentiation patterns as compared to their normal counterparts. For years, the molecular mechanisms of differentiation have been intensively investigated, but poorly understood. Differentiation begins shortly after the first few cell divisions that follow fertilization. Throughout development, and in adult organisms, the ability of a cell to proliferate is intimately connected to its state of differentiation. Adult tissues generally express a variety of factors fliat act to maintain both the proliferation and differentiation status of cells. These include secreted molecules, transmembrane receptors, intracellular signaling molecules and transcription factors. For example, myoD (81) and c/EBP-a (82) are nuclear factors that activate the transcription of muscle- and adipocyte-specific genes, respectively in addition, both proteins are potent inhibitors of cell proliferation. 

The helix-loop-helix motif appears to be another way of creating heterodimers that can bind to asymmetric sites on the DNA. Like the leucine zipper proteins, the helix-loop-helix proteins have a basic region that contacts the DNA and a neighboring region that mediates dimer formation. Based on sequence patterns, it has been proposed that this dimerization region forms an a helix, a loop, and a second a helix. Like the leucine zipper protein, the activity of the helix-loop-helix proteins is modulated by heterodimer formation. For example, the MyoD protein, which appears to be the primary signal for differentiation of muscle cells, binds DNA most tightly when it forms a heterodimer with the ubiquitously expressed E2A protein. 

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