All types of cells are capable of receiving signals from their environment and mounting an appropriate response to the signal, such as chemotaxis toward a nutrient source or toward other cells emitting a pheromone. The key difference between microorganisms and more-complex plants and animals is that the former are largely independent, with each cell in contact with the environment. In contrast, more complex plants and animals are self-contained entities whose interior is mostly insulated from the environment. Animals have complex organ systems, with each organ specialized for a particular function. Therefore, the survival of the organism depends on the precise regulation of growth, differentiation, and metabolism in different groups of cells throughout the animal.

A steroid hormone binds to a two-part receptor within the cell. This links with a coactivator, making a complex that binds to DNA. This triggers transcription of the target gene, through interaction...