The RhoH/TTF (ARHH) gene encodes a new member of the Ras superfamily of small GTPases. The gene was identified by fusion to the BCL6/LAZ3 oncogene in an initially described t(3;4)(q27;p11) translocation in a non-Hodgkin's lymphoma cell line. The predicted amino acid sequence of the RhoH/TTF gene product includes Rho-like GTPase structural motifs. The RhoH/TTF gene is restrictively expressed in hematopoietic cells and tissues. Mutations in the human RAS genes have been shown previously to be tumorigenic; in the search for a potential implication of the RhoH/TTF gene in hemopoietic malignancies, we established its genomic structure. The RhoH/TTF gene spans 35 kb and contains two exons, with the second bearing the entire amino-acid-coding region. Chromosomal mapping, by FISH experiments, places the RhoH/TTF gene on the short arm of chromosome 4, band p13.