Figure 4
From: Accuracy and quality of massively parallel DNA pyrosequencing
The presence of Ns correlates well with sequencing errors. The presence of an N in the sequence indicates the GS20's inability to accurately call a base at that position within the sequence. The number of other sequencing errors (substitutions, insertions and deletions) within a sequence read correlates with number of uncalled bases. By removing all reads that contain one or more Ns, the overall sequencing error rate drops substantially.