Small G proteins switch from a resting, GDP-bound state to an active, GTP-bound state. As spontaneous GDP release is slow, guanine-nucleotide-exchange factors (GEFs) are required to promote fast activation of small G proteins through replacement of GDP with GTP in vivo . Families of GEFs with no sequence similarity to other GEF families have now been assigned to most families of small G proteins. In the case of the small G protein Arf1, the exchange of bound GDP for GTP promotes the coating of secretory vesicles in Golgi traffic . An exchange factor for human Arf1, ARNO , and two closely related proteins, named cytohesin 1  and GPS1 , have been identified. These three proteins are modular proteins with an amino-terminal coiled-coil, a central Sec7-like domain and a carboxy-terminal pleckstrin homology domain. The Sec7 domain contains the exchange-factor activity . It was first found in Sec7, a yeast protein involved in secretion , and is present in several other proteins, including the yeast exchange factors for Arf, Gea1 and Gea2 [7-9]. Here we report the crystal structure of the Sec7 domain of human ARNO at 2 Angstrom resolution and the identification of the site of interaction of ARNO with Arf.