HTRA1 and preeclampsia: Furthermore, there is a body of evidence that some HTRA (High temperature requirement protease A) serine proteases including HTRA1, HTRA2, HTRA3 and HTRA4, which are normally expressed in placental villi, are impaired in pathological pregnancies such as preeclampsia, intrauterine growth restriction, and spontaneous preterm birth, suggesting that some of these proteins may be used as early markers of pregnancy outcome [44].