Protein ubiquitylation is an evolutionarily conserved post-translational covalent modification that forms an isopeptide bond between the activated ubiquitin and the target protein [1].It is consisted of three-step enzymatic processes involving El ubiquitin-activating enzymes, ubiquitin-conjugating enzymes (E2s), and E3 ubiqutin ligases (E3s).E3s contribute to the specificity of this system by selectively targeting a particular target(s) and to the regulation of many cellular physiological processes such as cell proliferation, apoptosis and differentiation.Aberrant expression or dysfunction of E3s results in autoimmune diseases, tumorigenesis, and neurodegenerative disorders.However, it is not until recently that some E3s are recognized to function in the homeostasis of hematopoietic stem cells (HSCs) [2-5].
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