Transcription factors such as the glucocorticoid receptor and the CREB protein form homodimers and activate transcription. Other transcription factors form heterodimers. For example, a transcription factor known as myogenic bHLH forms a heterodimer with a protein called the E protein. This heterodimer activates the transcription of genes that promote muscle cell differentiation. However, when myogenic bHLH forms a heterodimer with a protein called the Id protein, transcriptional activation does not occur. Which of the following possibilities best explains this observation? Only one possibility is correct.

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Myogenic bHLH E Protein Id Protein Possibility 1 DNA-binding domain: Leucine zipper Yes No No Yes No Yes Possibility 2 DNA-binding domain: Leucine zipper: Yes Yes Yes No Yes Yes Possibility 3 DNA-binding domain:Yes Leucine zipper No Yes Yes No 0