UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GlcNAc-T I; EC 2.4.1.101) is a medial-Golgi enzyme essential for the synthesis of hybrid and complex N-glycans. We have isolated two overlapping genomic DNA clones which span 18 kilobases (kb) containing a single 2.5 kb exon for GlcNAc-T I. The exon includes most of the 5'-untranslated region, the complete coding sequence (1335 bases) for GlcNAc-T I (445 amino acids) and the complete 3'-untranslated region. The remaining exon (or exons) is at least 2.0 kb upstream of the intron-exon junction. Transient transfection of either clone into Lec 1 Chinese hamster ovary cell mutants (which lack GlcNAc-T I) indicates the presence of a promoter responsible for expression of a truncated transcript. Southern blot analysis indicates that the gene exists in single copy in the human genome and is located on chromosome 5. The human and rabbit enzymes are 85% similar at the nucleotide sequence level and 92% similar at the amino acid sequence level.