The Ras-related superfamily of guanine nucleotide binding proteins includes the R-Ras, Rap, Ral/Rec and Rho/Rab subfamilies. Increasing data suggests an important role for Rab proteins in either endocytosis or in biosynthetic protein transport. The process of transporting newly synthesized proteins from the endoplasmic reticulum to various stacks of the Golgi complex and to secretory vesicles involves the movement of carrier vesicles and requires Rab protein function. Rab proteins are also an integral part of endocytic pathways. Rab 23, also known as HSPC137, is a 237 amino acid member of the Rab family of proteins and localizes to the cytoplasmic side of the cell membrane. Rab 23 is believed to play a role in intracellular protein transportation and signal transduction mediated by small GTPases. Mutations in the gene encoding Rab 23 may result in Carpenter syndrome, also known as ACPS2 (acrocephalopolysyndactyly type 2), a condition characterized by obesity, cardiac defects, polysyndactyly and craniosynostosis.