Colony-stimulating factors (CSFs) are secreted glycoproteins which
bind to receptor proteins on the surfaces of hemopoietic stem cells
and thereby activate intracellular signaling pathways which can cause
the cells to proliferate and differentiate into a specific kind of
blood cell (usually white blood cells, for red blood cell formation
see erythropoietin).
http://www.biosolutions.info/2010/10/colony-stimulating-factor.html