Proposed secondary structure of the TFPI and the position of the amino acid exchange. The mature protein consists of 276 amino acids, forming three tandem Kunitz-type proteinase inhibitory domains, two connecting chains, an acidic N-terminus with negatively charged amino acids (−) and a basic carboxy-terminal end, containing a cluster of positively charged amino acids (+).5 6 The Pro¹⁵¹ to Leu¹⁵¹ change is located near the Kunitz-type inhibitor domain δ2 within the second connecting chain (magnification).