The mitogen-induced gene, GEM, encodes a GTP-binding protein that belongs to a new family within the Ras superfamily. The regulated expression pattern of Gem suggests a role for this protein in cellular responses to growth stimulation. To facilitate the assessment of the possible role of GEM in heritable and spontaneous disease processes, the genomic organization of human GEM and the chromosomal localization of human and murine GEM have been determined. GEM has been localized to the long arm of human chromosome 8 (8q13-q21) between the D8S85 and CA2 loci by genetic linkage analysis using an MspI restriction fragment length polymorphism within GEM . No consistent somatic chromosomal alterations or heritable diseases are associated with this region. Mouse Gem maps to the proximal region of chromosome 4 between Mos and Cga. To gain insight into the transcriptional regulation of GEM, we have established the transcription initiation site of GEM in human T cells and defined a 5′ upstream region sufficient for mitogen-responsive, inducible transcription.