IGF-1 LR3 (Long R3 IGF-1) is an 83-amino-acid recombinant analog of human insulin-like growth factor 1, expressed in E. coli. It differs from the native 70-residue protein in two ways: an arginine replaces glutamate at position 3 of the IGF-1 sequence (the “R3”), and a 13-residue N-terminal extension derived from porcine growth hormone remains attached (the “Long”). It has the molecular formula C₄₀₀H₆₁₉N₁₁₁O₁₁₅S₉, a molecular weight of approximately 9,111.5 g/mol with its three disulfide bonds formed, and CAS number 143045-27-6. In the primary literature the Arg3 substitution is reported to greatly reduce binding to IGF-binding proteins while largely preserving binding to the type-1 IGF receptor. Supplied as a white lyophilized powder, reconstituted in dilute acetic acid, stored refrigerated at 2–8 °C.