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Qualimap: intronic reads

I am using qualimap to assess the quality of my alignments.

However for most samples I am having around 12% of intronic reads. I know for a fact I have no gDNA contamination. What is causing this to happen?

alignment

How about intergenic reads? Why is it bad to have 50% intergenic reads?

Perhaps this wasn't polyA enriched, in which case that would be quite reasonable.

1 answer

This is due to a combination of pre-mRNA, intron inclusion, and unannotated exons.

Is there a correlation between this and the lack of sample clustering in a PCA plot?

There can be, particularly if there's a large degree of variability between the samples.

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