Gene expression is primarily determined by various factors such as chemicals in the body, environmental influences, and regulatory mechanisms. While the brain can have an impact on gene expression, especially in relation to behavior and stress responses, it is not a direct determinant of gene expression at the molecular level. Senescence refers to the process of aging, which can influence gene expression patterns over time. Protein synthesis is the result of gene expression rather than a determinant of it.
In summary, the most relevant choices from the options provided are "chemicals in the body" and "senescence," as they can both influence how genes are expressed.