allows the hormone to exert its effect, In that way, the hormone can circulate to all tissues while affect ing a few those with the specific receptor Hormones can be divided into several classes based on chemical makeup: amino acid derivatives, protei and The chemical is ture influences the way in which the transported in the blood and the manner in which it exerts its eftect on the tissue. For example, while steroid hormones lipid-like structure requires that they be transported bound to plasma protein (to "dis- solve in the plasma) that same lipid-like structure allows them to diffuse through cell membranes to exert their effects This is discussed in detail in a later Altering Activity of DNA in the Nucleus." Hormones exist in very small quantities in the blood and are measured in microgram (10-6 g), nanogram (10-9 g), and picogram (10 12 g) amounts. It wasn't until the 1950s that analytical techniques were im proved to the point of allowing measurement of these low plasma concentrations (94).