In this report we describe the chromosome mapping and genomic organization of the human Gal beta 1,3GalNAc/Gal beta 1,4GlcNAc alpha 2,3-sialyltransferase gene. The gene is localized to human chromosome 11(q23-q24) by in situ hybridization of metaphase chromosomes. It spans more than 25 kilobases of human genomic DNA and is distributed over 14 exons that range in size from 61 to 679 base pairs. Previous characterization of cDNAs encoding the Gal beta 1,3GalNAc/Gal beta 1,4GlcNAc alpha 2,3-sialyltransferase revealed that the gene produces at least three transcripts in human placenta, which code for identical protein sequences except at the 5' ends (Kitagawa, H., and Paulson, J. C. (1994a) J. Biol. Chem. 269, 1394-1401). Repeated screening for clones that contain the 5' end of the cDNA has identified two additional distinct mRNAs that are expressed in human placenta. Comparison of the genomic DNA sequence with that of the five different mRNAs indicates that these transcripts are produced by a combination of alternative splicing and alternative promoter utilization. Northern analysis indicated that one of them is specifically expressed in placenta, testis, and ovary, indicating that its expression is independently regulated from the others.