Sterol regulatory element binding protein-1 (SREBP1) and SREBP2 are structurally related proteins that control cholesterol homeostasis by stimulating transcription of sterol-regulated genes, including those encoding the low-density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl CoA synthase. SREBP1 and SREBP2 are 47% identical, and they share a novel structure comprising a transcriptionally active NH2-terminal basic helix-loop-helix-leucine zipper (bHLH-Zip) domain followed by a membrane attachment domain. Cleavage by a sterol-regulated protease frees the bHLH-Zip domain from the membrane and allows it to enter the nucleus. SREBP1 exists in several forms, possibly as a result of alternative splicing at both the 5' and the 3' ends of the mRNA. The genes for SREBP1 (SREBF1) and SREBP2 (SREBF2) have not been studied. In this paper we describe the cloning and characterization of the human SREBF1 gene. The gene is 26 kb in length and has 22 exons and 20 introns. The 5' and 3' sequences that differ between the two SREBP1 cDNAs are encoded by discrete exons, confirming the hypothesis that they result from alternative splicing. The chromosomal locations of human SREBF1 and SREBF2 were determined by analysis of human-rodent somatic cell hybrids and fluorescence in situ hybridization. The SREBF1 gene mapped to the proximal short arm of chromosome 17 (17p11.2), and the SREBF2 gene was localized to the long arm of chromosome 22 (22q13).