RABGEF1 forms a complex with rabaptin-5 (RABPT5; MIM 603616) that is required for endocytic membrane fusion, and it serves as a specific guanine nucleotide exchange factor (GEF) for RAB5 (RAB5A; MIM 179512) (Horiuchi et al., 1997 [PubMed 9323142]).[supplied by OMIM, Mar 2010].