For each target region, the mean coverage of its bases is calculated as well as the percentaje of Gs and Cs it contains. For each target region, a point (GCcontent,Meancoverage) is painted in the graph. This drawing allows to observe sequencing biases which depend on the GC content of target regions. For example, lower coverage in sequencing regions with high GC or high AT content has long been observed. GC bias in sequencing studies is in large part due to early PCR steps during library generation where high and low GC content cause reduced amplification and therefore lower sequencing coverage.