The three-dimensional structure of full-length human BPI (456 amino acids) as determined by X-ray crystallography. The N-terminal domain of the protein is shown in magenta, and the C-terminal domain in purple. A 21-residue linker connecting the two domains is shown in yellow. The recombinant N-terminal BPI constructs testing in clinical trials correspond to residues 1-193, comprising nearly the entire N-terminal domain of the protein.