Figure 1: Selective pressure for HSCs to engraft enriches for clones with proviral integration sites that confer an engraftment advantage. After harvesting patient HSCs the cells are transduced with retroviral vectors, leading to a polyclonal population of cells with numerous different proviral insertion sites. Following transfusion of the cells into the patient, cells with insertions near genes that confer a competitive engraftment advantage (red, purple clones) will become enriched. Provirus vector integration sites in the purple and red cells are thus over-represented.