Retroviral integration and gene density. Integration sites (□) and integration hot spots (■) of RV (A) and LV (B) vectors in CD34⁺ cells are plotted according to the number of known genes contained in a range of 1 Mb around each insertion site, in intervals of 5 genes/Mb. The distribution of control sequences is indicated by ■. ■ represents the frequency of 1-Mb segments in the human genome for each gene density interval. n values indicate the number of independent hits in each group.