TPO and hematologic disorder: Clinical gene transfer using retroviral vectors for hematological disorders typically involves isolation of CD34+ cells, transduction of the cells with the vectors in the presence of recombinant cytokines, such as throbopoietin (TPO), stem cell factor (SCF), Fms-like Tyrosine Kinase-3 ligand (FLT3L) and interleukin-3, and transfusion of the cells back to patients.