Schematic outline of the experimental strategy.

<p>Freshly isolated CD34⁺ umbilical cord blood cells were separated into four experimental groups: Group 1: non treated CD34⁺ cells analyzed immediately; Group 2: cells stimulated with cytokines only; Group 3: cells cultured with cytokines and undergoing a mock infection with polybrene only but no LV; Group 4: cells cultured with cytokines, then transduced with polybrene and LV. The cells were either fixed for immunochemical analysis or were frozen for DNA extraction and methylation analysis. In a typical experiment, cells isolated from the same cord blood were split into the different groups. For immunochemical analysis groups of cells deriving from a single cord were compared. For methylation analyses, DNA from several cord blood donors was pooled.