We have previously reported that the expression of HC gp-39, a 39-kDa secretory glycoprotein and member of the chitinase protein family, is associated with late stages of monocyte to macrophage maturation. To allow further investigations of its unique expression pattern and to facilitate studies on the regulation of this "marker gene," we characterized the genomic organization of the HC gp-39 gene (HGMW-approved symbol CHI3L1) and cloned its 5'-proximal promotor region. The gene is composed of 10 exons distributed over an 8-kb region. Two transcriptional initiation sites located 82 and 126 nucleotides upstream of the ATG start codon were identified using primer extension and S1 nuclease protection assays. We sequenced 1.3 kb of the 5'-flanking region and identified a variety of putative regulatory elements.