Hormones are certain types of molecules that are found in nature that act as signallers. They are transported around a living being to change the behaviour or the physiology of an individual. A typical example is oestrogen, which is found in all mammals. This regulates many reproductive functions, especially in females. Although their chemical nature differs greatly depending on the particular hormone, they fall into three main categories. Eicosonads are an enzyme form of hormone while steroids tend to make up cell structures and are found in fungi and plants as well as animals. Thirdly, there are amino acids which are hormones that are derived from protein forms.