Properly the study of endocrinology, but often provide good molecular models for the control of gene expression. Generally, in animals, cells that are already determined may not express that determination until stimulated by a hormone. In plants, there are cases where hormones appear to induce developmental states such as determination. The ratio of auxins to cytokinins, for example, controls whether tissue cultures produce roots or stems or both. Amines, peptides, and glycoprotein hormones often act by a mechanism like this one.
G proteins mediate communication between receptor and effector inside the cell.