The RAB5 protein is a small GTPase involved in membrane trafficking in the early endocytic pathway. The protein encoded by this gene binds the GTP-bound form of the RAB5 protein preferentially over the GDP-bound form, and functions as a guanine nucleotide exchange factor for RAB5. The encoded protein is found primarily as a tetramer in the cytoplasm and does not bind other members of the RAB family. Mutations in this gene cause macrocephaly alopecia cutis laxa and scoliosis (MACS) syndrome, an elastic tissue disorder, as well as the related connective tissue disorder, RIN2 syndrome. Alternative splicing results in multiple transcript variants.