Cosmid clones containing the gene for human adenosine deaminase (ADA) were isolated. The gene is 32 kb long and split into 12 exons. The exact sizes and boundaries of the exon blocks including the transcription start sites were determined. The sequence upstream from this cap site lacks the TATA and CAAT boxes characteristic for eukaryotic promoters. Nevertheless, we have shown in a functional assay that a stretch of 135 bp immediately preceding the cap site has promoter activity. This 135-bp DNA fragment is extremely rich in G/C residues (82%). It contains three inverted repeats that allow the formation of cruciform structures, a 10-bp and a 16-bp direct repeat and five G/C-rich motifs (GGGCGGG) disposed in a strikingly symmetrical fashion. Some of these structural features were also found in the promoter region of other genes and we discuss their possible function. Knowledge of the exact positions of the intron-exon boundaries allowed us to propose models for abnormal RNA processing that occurs in previously investigated ADA-deficient cell lines.