NOS3 and Infertility: In this study, we found that eNOS derived NO, is expressed in different areas of endometrial tissues of women with unexplained infertility as of normal endometrium but inconsiderable differences between expressions of eNOS in two groups except in luminal epithelium suggest that endothelial isoform of nitric oxide synthase may not mainly be involved in the process of preparation of endometrium for implantation in unexplained infertility; also endocrine factors acting on stromal cells in this stage would not effected enzyme expression (14).