The ALS2 gene encodes alsin, a GEF, that has dual specificity for Rac1 and Rab5 GTPases. Alsin mutations in the form of truncated proteins are responsible for motor function disorders including juvenile-onset amyotrophic lateral sclerosis, familial juvenile primary lateral sclerosis, and infantile-onset ascending hereditary spastic paralysis. The alsin protein is widely expressed in the developing CNS including neurons of the cerebral cortex, brain stem, spinal cord, and cerebellum. Alsin contains a regulator of chromosome condensation 1 (RCC1) domain, a Rho guanine nucleotide exchanging factor (RhoGEF) domain, a PH domain, a Membrane Occupation and Recognition Nexus (MORN), a vacuolar protein sorting 9 (Vps9) domain, and a Dbl homology (DH) domain. Alsin interacts with Rab5 through its Vps9 domain and through this interaction modulates early endosome fusion and trafficking. The GEF activity of alsin towards Rab5 is regulated by Rac1 function. The GEF activity of alsin for Rac1 occurs via its DH domain and this interaction plays a role in promoting spinal motor neuron survival via multiple Rac-dependent signaling pathways. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N-terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes.