text: user1369: I would be grateful if someone could take a quick look at these FASTQC results. This is rna-seq paired-end data. From the FASTQC manual, an unusual distribution seems to be suggestive of contamination and a shift in the curve is suggestive of a systematic bias. GC content distribution both pre-alignment and post-alignment are strange. 
  Samples are paired end, strand specific and % of mapped reads is above 95% for all the samples. There is no adapter content also. 
  What could be the problem?
  
  
  
  user73: Although generally I recommend people to be wary about the warnings that FastQC gives (it tends to be overly paranoid), the GC content graphs here do look odd. It's good that you've had a look at it with another program to confirm the observation. They should have a distribution that is fairly close to normal, a bit like this:
  
  Without more information about your sample, I can't tell whether or not this is expected from your input. This looks like fairly substantial contamination, but could also be caused by biased selection during sample preparation (e.g. low quality sample, low yield, PCR amplification bias), or just looking at a multi-organism sample rather than a single-organism sample.
  As a contamination check, I'd recommend running your RNASeq reads through a kmer-based classification tool (e.g. OneCodex, Centrifuge, BBSketch, or Sourmash). This should tell you the most likely place that contaminant sequences came from.
  
original_text: 1369: <p>I would be grateful if someone could take a quick look at these FASTQC results. This is rna-seq paired-end data. From the FASTQC manual, an unusual distribution seems to be suggestive of contamination and a shift in the curve is suggestive of a systematic bias. GC content distribution both pre-alignment and post-alignment are strange. </p>
  
  <p>Samples are paired end, strand specific and % of mapped reads is above 95% for all the samples. There is no adapter content also. </p>
  
  <p>What could be the problem?</p>
  
  <p><a href="https://i.stack.imgur.com/ZAO4g.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZAO4g.jpg" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/ZwwdL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZwwdL.png" alt="enter image description here"></a></p>
   73: <p>Although generally I recommend people to be wary about the warnings that FastQC gives (it tends to be overly paranoid), the GC content graphs here do look odd. It's good that you've had a look at it with another program to confirm the observation. They should have a distribution that is fairly close to normal, a bit like this:</p>
  
  <p><a href="https://i.stack.imgur.com/Hwnvx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Hwnvx.png" alt="GC content from mouse cDNA"></a></p>
  
  <p>Without more information about your sample, I can't tell whether or not this is expected from your input. This looks like fairly substantial contamination, but could also be caused by biased selection during sample preparation (e.g. low quality sample, low yield, PCR amplification bias), or just looking at a multi-organism sample rather than a single-organism sample.</p>
  
  <p>As a contamination check, I'd recommend running your RNASeq reads through a kmer-based classification tool (e.g. <a href="https://www.onecodex.com/" rel="nofollow noreferrer">OneCodex</a>, <a href="https://ccb.jhu.edu/software/centrifuge/manual.shtml" rel="nofollow noreferrer">Centrifuge</a>, <a href="https://www.biostars.org/p/234837/" rel="nofollow noreferrer">BBSketch</a>, or <a href="https://github.com/dib-lab/sourmash" rel="nofollow noreferrer">Sourmash</a>). This should tell you the most likely place that contaminant sequences came from.</p>
  