This gene encodes a protein that has been shown to function as a guanine nucleotide release factor in mouse and to regulate the expression and function of the Nav1.5 cardiac sodium channel in human. Alternative splicing results in multiple transcript variants. May regulate the intracellular trafficking of RAN. Promotes guanine nucleotide release from RAN and inhibits binding of new GTP by preventing the binding of the RAN guanine nucleotide exchange factor RCC1. Regulates the levels of GTP-bound RAN in the nucleus, and thereby plays a role in the regulation of RAN-dependent mitotic spindle dynamics (By similarity). Enhances the expression of SCN5A at the cell membrane in cardiomyocytes.