Structure of the cytoplasmic domain of WT G-CSF-R and the naturally occurring truncated mutant. The cytoplasmic domain of WT hG-CSF-R contains 3 cytokine receptor superfamily homology regions, designated box 1, box 2, and box 3 (gray boxes). The transmembrane domain of the receptor is represented as a black box. There are 4 tyrosine residues (Y) in the carboxy-terminal domain of G-CSF-R. The IL-3–dependent murine myeloid 32D cell line was transfected using a pLNCX retroviral expression vector containing human WT hG-CSF-R or the hG-CSF-R mutant (Δ715) cDNA. The putative ITAM-like motif, containing tyrosines 727 and 747, is indicated by the dark-gray box.