X-ray structure of human leptin-E100. This mutant form of leptin (W100E) has comparable biological activity to the wild-type protein but crystallizes more readily. The protein, which is colored in rainbow order from its N-terminus (blue) to its C-terminus (red), forms a four-helix bundle, as do many protein growth factors (e.g., human growth hormone; Fig. 13-3). The sidechains of Cys residues 96 and 146, which form a disulfide bond, are shown in ball and stick mode with S yellow and C green. Residues 25 to 38 are not visible in the X-ray structure. [Based on an X-ray structure by Faming Zhang, Eli Lilly & Co., Indianapolis, Indiana. PDBid 1AX8.]