Structure of the human IL-12 p35 gene (A and B). The λ Fix II clone contained three BamHI fragments of approximately 3, 6, and 9 kb, respectively, from the 5′ end. Restriction sites for BamHI (B), EcoRI (E), andPst I (P) are noted. Exons are labeled numerically (I through VII). The transcription start sites (see Fig 5) are labeled as S1 (monocyte start site) and S2 (lymphoblastoid cell start site). Exon boundaries were sequenced from within the exons, and intron distances were estimated by PCR using appropriate exon primers or were determined directly by sequencing through to the next exon. Numbers surrounding the exon boxes (B) refer to amino acids, beginning with the second methionine codon (Met35 in Wolf et al21). Sequence data from this clone are available from GenBank under accession no.AF050083. (C) Comparison of human and mouse (accession no.S82412) p35 gene 5′ flanking sequences. Nucleotide sequences were aligned using GCG BestFit, as described in the text. Sequences are aligned from the 3′ end of exon I (human), which represents the second exon in the mouse. Notations are made for the two transcription start sites in the human gene (S1, the monocyte start site; S2, the lymphoblastoid start site). The TATA box (TATAAA) is overlined, as are the two methionine (ATG) codons Met1 and Met35. The proposed 5′ end of the second exon of the mouse p35 gene is noted; the mouse sequence presented in this figure therefore represents the 3′ end of the first intron and the entire second exon. There is no evidence for an upstream first exon in the human gene.