Schematic illustration of CSF3R truncation mutations and the functional consequences. CSF3R truncation mutations at 658-685 which abrogate STAT5 activation do not transform BaF3 cells and do not respond to G-CSF, even when combined with T618I mutation. Truncation mutations at 700-738 do not transform Ba/F3 cells alone, however, these truncations can transform Ba/F3 when combined with T618I and are hyposensitive to G-CSF due to the disruption of the mitogenic enhancing domain. Truncation mutations between 738-791 and 793-823 transform Ba/F3 and induce G-CSF hypersensitivity due to the interruption of the internalization and the de-phosphorylation domain respectively. MED: mitogenic domain; D: Domain.