The Cyp4b1 structural gene comprises 12 exons that span similar to23 kb. The transcriptional initiation site, mapped by 5'-RACE and primer extension analyses from kidney mRNA, was found to be 34-bp upstream of the translation initiation site. Cyp4b1 mRNA expression was found restricted to kidney and androgenic up-regulation in several mouse strains was observed. Spatial location investigated by in situ hybridisation revealed abundant and specific expression of Cyp4b1 mRNA in the proximal tubules of the renal cortex. A 1.8-kb murine Cyp4b1 5'-flanking region, which encompasses a TATA box-like sequence and several putative transcription factor-binding sites, was isolated. Transient transfection studies with different Cyp4b1 promoter constructs indicated that this flanking region exhibits promoter activity when expressed in different cell lines and that a 98-truncated fragment represents the minimal sequence required for basal transcription. Androgen responsiveness was examined by cotransfection with an androgen receptor expression vector, in the presence of androgens. (C) 2003 Elsevier Science (USA). All rights reserved.