Human IGF-BP1 is a cysteine-rich secreted protein expressed in liver, decidual, and kidneys and is the most abundant IGF-BP in amniotic fluid. Levels of IGF-BP1 in serum are lowest after food. IGF-BP1 binds both IGF-I and IGF-II with equal affinity. Phosphorylated IGF-BP1 hinders IGF actions, whereas nonphosphorylated IGF-BP1 is stimulatory.