Figure 2. Comparison between STAT5 and Mpl knockout phenotypes. While c-Mpl deficiency causes severe megakaryocyte and platelet production defects, most other lymphoid and myeloid lineages are unaffected. Interestingly, Mpl knockout also selectively impairs early HSC/HPC function as determined by analysis of peripheral blood chimerism following transplantation. STAT5 knockout mice share the common HSC/HPC level defects, do not have intrinsic megakaryocyte/platelet defects, but have severe combined immunodeficiency that is unique to the role of STAT5 downstream of the common gamma chain and JAK3.