Neuropilin's a protein receptor active in neurons. Overview There're two forms of Neuropilins, NRP-1 and NRP-2. They're transmembrane glycoproteins, and predominantly co-receptors for another class of proteins known as Semaphorins. Of the Semaphorins, NRP-1 and NRP-2 are specifically receptors for Class-3 Semaphorins, which, amongst many things, are responsible for axon guidance during the development of the nervous system in vertebrates. Neuropilins work as co-receptors as they've a very sma… (More on Neuropilin)