Schematic diagram of the cytoplasmic domain of the granulocyte colony stimulating factor receptor. Depicted are the regions that mediate signals for proliferation and maturation as well as the 4 tyrosine residues that are phosphorylated on ligand binding. The critical region containing the mutations in severe congenital neutropenia is bracketed; the asterisk denotes the site for the alternative splicing event that generates the class IV isoform.