Any of several related proteins that act as long-range chemical cues to guide the growth of axons in vertebrates. Netrins are secreted and diffuse through the extracellular matrix, creating a concentration gradient that influences how the growth cone of the developing axon navigates through the tissues. Specific netrin receptors on the growth cone bind the netrin molecules. In some cases the elongating axon grows towards the source of the netrin, whereas in other cases it is repelled by the netrins, depending on the type of nerve cell involved.