in the posterior and lateral hypothalamus in mice showed a sixty-fold higher expression in the posterior hypothalamus. This higher expression is due to the presence of peripherin in the tuberomammillary neurons of the mouse posterior hypothalamus. Function The diverse properties of intermediate filaments, compared with the conserved microtubule and actin filament proteins, could be responsible for the distinguishing molecular shapes of different cell types. In nerve cells, for example, the expressions of different types of IFs relates to the change in shape during development. Early stages of development in neurons is marked by the outgrowth of neurites and axons contributing to the cells asymmetric shape. During these transitions in cell shape, only homopolymer type III intermediate filaments, such as those with peripherin, are made. As the nerve cell matures, these type III IFs are replaced by more complex type IV neurofilaments expanding the diameter of axons in order to attain normal velocities of action potentials. The exact function of peripherin is unknown. Expression of peripherin in development is greatest during the axonal growth phase and decreases postnatally, which suggests a role in neurite elongation and axonal guidance during development. Expression is also increased after axonal injury, such as peripheral axotomy in motor neurons and dorsal root ganglia. This upregulation implies that peripherin may also play a role in axon regeneration. However, experiments using peripherin depleted PC12 cells and peripherin knockout mice provide proof that the majority of neurons have no requirement of peripherin for axonal guidance and regrowth. PC12 cells lacking peripherin showed no defects in neurite outgrowth and peripherin knockout mice develop normally with no anatomical abnormalities or different phenotypes. In these experiments, peripherin deficiency did produce an upregulation of α-internexin, indicating the possibility that this type IV intermediate filament makes up for the loss of peripherin. Future studies of double knockout mice for both the peripherin and α-internexin genes might address this theory. However, while most peripherin knockout mice displayed normal neuron growth, its absence did affect development of a subset of unmyelinated sensory axons. In such mice, there was a "34% reduction in the number of L5 unmyelinated sensory fibers that correlated with a decreased binding of the lectin IB4." Gene (PRPH) The complete sequence of the human (GenBank L14565), rat (GenBank M26232) and mouse (EMBL X59840) peripherin genes (PRPH) have been reported and complementary DNAs (cDNA) thus far described are those for rat, mouse and Xenopus