## 4 Discussion
Owing to high levels of QSOX1 expressed in the seminal vesicle, we can purify the native QSOX1 protein from the seminal vesicle secretion. The machinery of the secretory pathway sophisticatedly folds the native QSOX1; thus, the antibody produced by this native glycoprotein possesses high specificity and sensitivity. QSOX1 can be detected even with relatively low levels in any tissue. QSOX1 is overexpressed in tumor cells of breast cancer [22] and pancreatic cancer [23]. QSOX1 was demonstrated to activate matrix metalloproteinase (MMP)-2 and MMP-9 activities on the extracellular matrix (ECM) to mediate cancer cell migration [22; 23]. Using the short hairpin RNA to knock down the highly expressed QSOX1 in the breast and pancreatic cancer cells significantly inhibited tumor cell invasion [22; 23]. These findings suggest high levels of QSOX1 may lead to tumor cell invasion and migration. In this study, we found that QSOX1 was highly expressed in the spinal cord, brainstem, lens, tooth, and placental trophoblasts. Some cells in these tissues are involved in cell migration during tissue development. For instance, motor neuron migration is involved in the organization of spinal motor neurons [24]. The development of neuron cells in brain tissues, including the midbrain, cerebellum, medulla oblongata, and pons, require the cell migration process [16]. Neural crest cell migration is involved in the development of teeth [25]. Lens morphogenesis needs lens fiber cell migration [26]. QSOX1 is also expressed at a moderate intensity in the epithelial cells of the vomeronasal organ. The embryonic vomeronasal structure equips with migrating neurons such as the endocrine gonadotropin-releasing hormone one neuron, which migrates from the nasal area to the brain area [27]. In addition, QSOX1 is highly expressed in the placenta, especially in trophoblast cells in the junction zone. At the maternal-fetal interface, the migration of trophoblast cells, facilitated by proteinases such as urokinase-type plasminogen activator and MMP-9, is crucial during embryo implantation and placentation [28]. Interestingly, QSOX1 mediates tumor cell invasion/migration by activation of MMP-2 and MMP-9 [23]. These findings might indicate that QSOX1 plays a vital role in cell migration during embryogenesis, but it is also possible that QSOX1 has other functions that need to be clarified by more experiments in the future. To investigate protein expression levels and their distribution, one should consider the spatiotemporal dynamical changes. The distinct expression levels and localization of QSOX1 described in reports may be caused by sampling at various times and spaces. Our research still has limitations. Serial sections only revealed the expression of QSOX1 in a small part of a tissue, which cannot fully understand the overall distribution of QSOX1 in the tissue under different times and spaces. Of course, the ideal situation is to analyze the genuine expression and distribution of QSOX1 in each tissue in various spatial and temporal conditions using longitudinal and transverse sections of the entire tissue. We acknowledge that this study may not have analyzed QSOX1 expression in all tissues. Nevertheless, there are some new findings, such as the expression in the teeth and vomeronasal organs, which were not found in previous studies. ECM is a complex and dynamic structure outside the cells, which plays a vital role in development, wound healing, and physiological and pathological processes [29]. QSOX1 activity has been demonstrated to be required to incorporate laminin into fibroblast ECM, and without QSOX1, ECM cell migration is impaired [30]. QSOX1 has also been found to activate the activity of MMP-2 and MMP-9 on ECM of cancer cells [23], suggesting that QSOX1 could modulate ECM properties and affect cell migration. Exogenous QSOX1 can induce vascular smooth muscle cell migration and proliferation and contribute to neointima thickening of the carotid artery in rats with balloon injury [31].