Gene expression is determined by several factors, including:
- Chemicals in the body: This refers to various hormones, signaling molecules, and other chemicals that can influence the transcription of genes.
While protein synthesis is a result of gene expression rather than a factor determining it, the brain's regulatory role can also impact gene expression through its control of hormone release and other signaling pathways. Senescence, on the other hand, can affect gene expression in the context of aging or cell division.
Overall, the most direct answer from the options provided would be chemicals in the body.