The two glial cell types responsible for myelination of axons have also been investigated in the context of ALS. Oligodendrocytes myelinate axons in the CNS whereas Schwann cells are responsible for myelination in the peripheral nervous system (PNS). Whilst they perform similar functions, there are also important differences between these two cell types. Schwann cells form a single myelin sheath around one single axon, whereas oligodendrocytes myelinate many different axons. Furthermore, there are differences in the protein composition of CNS and PNS myelin.