We provide a therapeutic
drug delivery polypeptide that can be used for treatment. [Solution] The present invention relates to a
protein suitable for use as a
scaffold to which a
target peptide binds, or a
protein contained within a complex to which a target agent binds. It also relates to a complex suitable for selectively delivering a complex of a target agent to specific types of cells and tissues. It also relates to nanoparticles containing such a complex. In one embodiment, a polypeptide is provided comprising (i) 11 β-chain domains designated A, B, C, D, E, F, G, H, I, J, and K, and (ii) 10 loop regions connecting two consecutive β-chain domains, designated AB, BC, CD, DE, EF, FG, GH, HI, IJ, and JK loops, wherein at least one of the loop regions is a homogeneous loop region variant, and at least one of the β-chain domains is a homogeneous β-chain variant.