 Differences in gene expression are especially clear within multicellular organisms , where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes . All the cells in a multicellular organism derive from a single cell , differentiating into variant cell types in response to external and intercellular signals and gradually establishing different patterns of gene expression to create different behaviors . As no single gene is responsible for the development of structures within multicellular organisms , these patterns arise from the complex interactions between many cells . 
