Before revision:  In general, their statistics and location are dominated by sequence heterogeneity. For genomic DNA , bubbles are frequently located directly upstream of transcription start sites.
After revision:  In general, their statistics and location are dominated by sequence disorder. Compared to random sequences with identical GC-content, genomic DNA has a significantly increased probability to open large bubbles under superhelical stress. These bubbles are frequently located directly upstream of transcription start sites.