The<unk> s <unk> s) are polypeptides with high sequence similarity to<unk> .<unk> s are part of a complex system that cells use to communicate with their physiologic environment.  This complex system (often referred to as the<unk>  "axis") consists of two cell-surface receptors <unk> 1R and<unk> 2R), two ligands <unk> -1 and<unk> -2), a family of six high-affinity<unk>  binding proteins <unk> BP 1-6), as well as associated<unk> BP degrading enzymes, referred to collectively as proteases.
Insulin-like growth factor
Gonadotropins
Bryostatin
Bone morphogenetic protein
Growth factor
Parathyroid hormone 
Menotropins
Antidiuretic hormone