The hemopoietic regulators--an embarrassment of riches.

A large, and growing, group of glycoprotein regulators is now recognized to control the proliferation, maturation and functional activity of the eight major families of blood cells. Each hemopoietic regulator is the product of multiple cell types and there is a puzzling redundancy of regulators able to stimulate each subfamily of hemopoietic cells. Each regulator is polyfunctional but it remains unclear how a single type of activated receptor is able to initiate the diverse cellular responses induced.