One aspect of hormones that is often largely neglected is the understanding of how they work. Hormones have no influence in the body without receptors, to which hormones bind in order to affect the cells where the receptors are located. Hormones can only bind to their own (or sometimes very similar) receptors, like keys that fit only into locks that are configured for them. There must also be a good match between the number of keys to the number of locks—a lot of keys and few locks are not effective, and a few keys but a lot of locks are not effective. The number of available receptors is dynamic—changeable—and is influenced by different factors for different hormones; for example, prolactin helps to stimulate the creation of estrogen receptors. During pregnancy and lactation, the number and binding ability of hormone receptors important for breast development and milk production change according to a combination of their genetically programmed timetables and these other influences.