## 1 Introduction  
Quiescion Q6 sulfhydryl oxidase 1 (QSOX1), a flavin adenine dinucleotide (FAD)-dependent sulfhydryl oxidase, was first purified from the rat seminal vesicle secretion and characterized to catalyze the oxidation of low molecular thiol compounds such as glutathione and 2-mercaptoethanol to disulfide bonds [1]. Sequence analyses and biochemical studies revealed that QSOX1 and egg white sulfhydryl oxidase belong to the FAD-dependent sulfhydryl oxidases [2]. [PERSON] et al. demonstrated that chicken egg white sulfhydryl oxidase utilizes an internal redox-active cysteine bridge and a FAD moiety, the same in the QSOX1, to oxidize low molecular weight thiols; the oxidase also exhibits a high catalytic activity toward a range of reduced peptides and proteins in vitro [3].  
QSOX1 has two main isoforms, i.e., long-form (-82 kDa) and short-form (-65 kDa) [4]. The long-form includes a transmembrane region, while the short-form without the transmembrane part is secreted from the cell. Thus, the short form exists in biological fluids such as serum [5], seminal plasma [6], milk [7], etc. QSOX1 has been demonstrated to be expressed ubiquitously in various normal tissues [6, 8] and is highly expressed in tumor cells such as breast, prostate, and pancreatic cancers [9]. Interestingly, the short form is the main QSOX1 form [6, 8, 9].  
QSOX1 is expressed in the seminal vesicle, vas deferens, epididymis, and testis in male reproductive tissues [6; 8; 10]. It is also present in human seminal plasma [6]. The secretory QSOX1 can already be purified from the rodent's seminal vesicle secretion [1; 6]. Its enzyme activity is involved in cross-linking seminal vesicle proteins to form the vaginal plug after mating [11]. It has also been detected in sperm aggregates in mouse uterus and human semen. Those agglutinated sperm are rich in thiols, oxidatively stressed, and apoptotic sperm. Therefore, QSOX1 may help to remove injured sperm [6].  
QSOX1 may have different distributions in the same tissue. For example, the expression of QSOX1 gradually increased from caput to cauda segment of the epididymis, while mouse sperm is mature in caudal epididymis, suggesting its role in sperm maturation [10]. Therefore, unraveling the detailed spatial and temporal expression of QSOX1 may help understand the physiological functions that QSOX1 may play in the tissue.  
Guinea pig QSOX1 is expressed in the uterine epithelium during the estrus cycle. Because of its sulfhydryl oxidase activities, it is considered involved in the extracellular matrix modification during uterine differentiation and blastocyst implantation [12]. Although QSOX1 is present in the uterus [8], no report addressed QSOX1's expression in the pregnant uterus. QSOX1 is also present in the syncyotrophoblast of the human placenta [13] and has been suggested to regulate trophoblastic apoptosis in preeclampsia [14]; however, the dynamic changes of QSOX1 levels during the whole placentation process are still unknown. While a paper has been published on the expression of QSOX1 in the developing mouse embryo [15], we found some discrepancies, and a more comprehensive study is still necessary. Therefore, this study investigated the spatial and temporal expression of QSOX1 in the gestational uterus, placenta, and developing embryos of mice during pregnancy.