If your priority is building muscle, IGF 1 is what really matters. Recombinant Human LR3 IGF I IGF 1 GMP Protein, CF 8335D GMP: R&D Systems IGF 1 LR3 does not promote growth in late gestation growth restricted fetal sheep American Journal of Physiology Endocrinology and Metabolism American Physiological Society Modulatory activity of testosterone on growth pattern and IGF 1 levels in vanishing testis syndrome: a case report during 15 years of follow up BMC Endocrine Disorders Springer Nature Link What is IGF 1 LR3?, Insulin Like Growth Factor 1 Long R3 (IGF 1 LR3) is a modified version of IGF 1 with a longer half life, making it significantly more potent and effective for muscle growth,