PTI is a member of an emerging class of intracellular serpins.On the basis of its interaction with thrombin, through which it was discovered, the inhibitor was operationally named the placental thrombin inhibitor (PTI). Amino acid sequence comparisons suggested that its reactive center is located at arg341 and cys342, that it lacks a classic N-terminal signal sequence, and that it has the highest degree of similarity to intracellular serine proteinase inhibitors (serpins), such as the human monocyte/neutrophil elastase inhibitor (ELANH2). Analysis of human tissue mRNA indicated that PTI is expressed widely, with the highest levels in cardiac and skeletal muscle and placenta.Spi3, is developmentally regulated, and that, unlike the situation in the human, Spi3 is absent from the mouse placenta.