Comparison of RNA fingerprinting of the avian eggshell gland (ESG) without and with an egg revealed upregulation of a 382-bp cDNA fragment that showed high homology to the mammalian glypican 4 (GPC-4). The gene sequence revealed a conserved glypican signature, a glycosyl phosphatidyl inositol-anchorage site, and cystein residues, most of which were conserved. GPC-4 was expressed in the ESG in a circadian fashion only during the period of eggshell calcification, when maximal mechanical strain was imposed. Removal of the egg just before to its entry into the ESG, with consequent elimination of the mechanical strain, caused reduction in the gene expression. Artificial application of the mechanical strain induced expression of the GPC-4 gene that was related to the level of the strain. GPC-4 expression was strain dependent in other parts of the oviduct. In the ESG, GPC-4 was expressed exclusively by the glandular epithelium and not by the pseudostratified epithelium facing the lumen. In summary, we cloned the avian homologue of GPC-4, established its pattern of expression in the avian ESG, and demonstrated for the first time that this gene is regulated by mechanical strain.