['To analyze endometrial mRNA and protein expression of osteopontin and its receptor, beta3-integrin, throughout the menstrual cycle.', 'Study by immunohistochemistry, RNase protection assay, and ELISA.', 'Academic research unit.', 'Forty-five regularly cycling women without endometrial pathology.', 'Expression of endometrial osteopontin and beta3-integrin mRNA was analyzed by RNase protection assay in endometrium, endometrial epithelial cells, stromal cells, and endometrial leukocytes (CD45) and by immunohistochemistry in frozen sections of endometrium throughout the menstrual cycle. Concentration of osteopontin was studied in uterine secretions by ELISA.', 'mRNA and protein expression of osteopontin and beta3-integrin.', 'Osteopontin mRNA and protein was weakly expressed in the proliferative phase and maximally expressed in the mid to late secretory phases in endometrium, endometrial epithelial cells, and endometrial leukocytes and in uterine secretions. Beta3-integrin mRNA and protein were expressed in stromal cells throughout the menstrual cycle and were maximally expressed in epithelial cells in the mid to late secretory phases.'] Is endometrial osteopontin , a ligand of beta3-integrin , maximally expressed around the time of the `` implantation window ''?