SPP1 and Infertility: Here, decreased levels of OPN and CD44 were observed in the secretory endometrium of ovPCOS patients compared to fertiles, whereas levels of CD44, OPN and pro-inflammatory cytokines were increased in serum, as well as in the media used to collect endometrial biopsies suggesting that circulating CD44 and OPN are of endometrial origin in these infertile patients.