IGF1 and neoplasm: Circulating IGF-1 levels may reflect the IGF-1 expression from several organs/tissues and/or metabolic processes and may thus not correlate with BrCa status per se. Thus, tissue IGF-1 levels may be a better marker of tumor IGF-1 expression compared to serum levels, as has already been established in mammary gland branching morphogenesis [47].