The complete 4775-nt cDNA encoding the human serotonin 5-HT2c receptor (5-HT2cR), a G-protein-coupled receptor, has been isolated. It contains a 1377-nt coding region flanked by a 728-nt 5'-untranslated region and a 2670-nt 3'-untranslated region. By using the cloned 5-HT2cR cDNA probe, the complete human gene for this receptor has been isolated and shown to contain six exons and five introns spanning at least 230 kb of DNA. The coding region of the human 5-HT2CR gene is interrupted by three introns, and the positions of the intron/exon junctions are conserved between the human and the rodent genes. In addition, an alternatively spliced 5-HT2CR RNA that contains a 95-nt deletion in the region coding for the second intracellular loop and the fourth transmembrane domain of the receptor has been identified. This deletion leads to a frameshift and premature termination so that the short isoform RNA encodes a putative protein of 248 amino acids. The ratio for the short isoform over the 5-HT2CR RNA was found to be higher in choroid plexus tumor than in normal brain tissue, suggesting the possibility of differential regulation of the 5-HT2CR gene in different neural tissues or during tumorigenesis. Transcription of the human 5-HT2CR gene was found to be initiated at multiple sites. No classical TATA-box sequence was found at the appropriate location, and the 5'-flanking sequence contains many potential transcription factor-binding sites. A 7.3-kb 5'-flanking 5-HT2CR DNA directed the efficient expression of a luciferase reporter gene in SK-N-SH and IMR32 neuro-blastoma cells, indicating that it contains a functional promoter.