Tag clusters classified as positional and regional biases. The Venn diagram shows the number of tag clusters where start site selection is biased due to tissue. A biased tag cluster can either have distinct medians for tag starts for different tissues (termed positional bias) and/or have subregions that have a significantly different tag composition than the whole tag cluster. Most tag clusters that have positional bias also have subregions with significantly different tag composition. Only the tag clusters that are not classified as positional bias but include tissue-specific subregions are termed regional biases.