4-1BB is an inducible T cell surface receptor which belongs to the nerve growth factor receptor superfamily, a group of cysteine-rich cell-surface proteins. 4-1BB is a 30-kDa glycoprotein and exists as both a monomer and a 55-kDa dimer on the T cell surface. Cross-linking 4-1BB with monoclonal antibody resulted in the 2- to 10-fold enhancement of T cell proliferation. We have isolated and characterized 4-1BB genomic clones and have found that the 4-1BB gene contains two different 5' untranslated regions, which are used alternately to form the 4-1BB mRNA. The two 5' UTRs were encoded in the same chromosome and were separated from one another by an intron of approximately 2.5 kb. The entire gene spans approximately 13 kb of mouse chromosome 4. 4-1BB gene consists of 10 exons and 9 introns, in which there are two exons for 5' untranslated regions and 8 exons for coding region. Most of the putative functional domains were encoded by separate exons. 4-1BB extracellular domain contains four potential C6 (CXn C XX C XX CXn C Xn C) motifs, of which the first motif is partial and the third is distinct from those of nerve growth factor receptor or TNF receptor 1. A comparison of exon-intron organization among the genes of the nerve growth factor receptor family indicated that most C6 motif is interrupted by an intron.