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SeqReady guide · After sequencing

How to read your RNA-seq results report

A spreadsheet with 20,000 rows just landed in your inbox. Breathe. You only need three columns and two plots to know what you've got.

By Eric A., MSc Molecular BiologyNovember 20264 min read
After 4 minutes you'll be able to

Many provider reports are made with a tool called DESeq2. They look intimidating, but every row answers the same three questions about one gene.

01Three columns, three questions

log2FoldChange
+2.0
How big, and which way?

+1 means twice as high, −1 means half.

padj
0.003
Can I trust it?

Filter on this, not on pvalue. Below 0.05 is a common cut-off.

baseMean
850
Is it really expressed?

Very low values can fake big changes.

-3⅛×-2¼×-1½×0same+12×+24×+38×◀ lower in your grouphigher in your group ▶log2FoldChange.
Swipe sideways to see the whole figure →
Fold change is on a log2 scale: each step of 1 doubles or halves. A +3 gene is 8 times higher.

02The volcano plot in one look

Each dot is a gene. The further right or left, the bigger the change; the higher up, the stronger the evidence. Your interesting genes live in the two top corners.

how big the change is (log2 fold change) →how sure you can be →Up, and convincingDown, and convincing
Swipe sideways to see the whole figure →
Illustration. Most genes (grey) change little or have weak evidence; coloured dots changed a lot and convincingly.

03Look at the PCA plot first

Each dot is a sample. Replicates should sit together. One sample far from its group can change the whole gene list, so ask your provider about it before you dig in.

Looks healthy● control ● treatedAsk about this one● control ● treated
Swipe sideways to see the whole figure →
Illustration. Left: replicates group by condition. Right: one treated sample sits apart.
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04Three checks before you trust the list

  1. Which way is "up"?A positive fold change means higher in your group compared with the reference group. DESeq2 picks the reference alphabetically unless told otherwise. Quick test: find the gene you knocked down. If it says "up", the comparison is flipped.
  2. Ignore tiny giantsA gene with a baseMean of 3 can show a 16-fold change from a handful of reads. Prefer clearly expressed genes, or confirm by qPCR.
  3. Keep the full tablePathway tools need every gene, not just the top hits. Save the original file.

05Things that look wrong but aren't

Empty (NA) padj valuesDESeq2 leaves them empty on purpose for low-count or outlier genes.
Your gene count differs from a paperIt depends on the padj and fold change cut-offs. Compare those first.
A comparison is missingWith 3+ groups, reports may skip pairs. Just ask for it.
Zero significant genesNot a failure in itself. Check the PCA plot, then talk to your provider.

You don't need to be a bioinformatician to read this report. Three columns, two plots, three checks, and you'll know what your data is telling you.

Common questions

What's the difference between p-value and padj?

The p-value looks at one gene on its own. padj adjusts for the fact that you tested thousands of genes, so it controls how many of your "significant" genes are likely false. Use padj.

Can I use FPKM or TPM values to compare groups?

They're useful for looking at expression levels, but differential expression tools work on raw counts with their own normalization. Use the provider's DE table for group comparisons.

I got zero significant genes. Did something go wrong?

Not necessarily. It can mean a small effect, high variation between replicates, or too few replicates to detect changes. Check the PCA plot and talk to your provider or a bioinformatician.

About the author
Eric A.

MSc in Molecular Biology from the University of Debrecen, Hungary, with research on nanopore sequencing for hospital infection control. He has also worked on the provider side of NGS services, helping universities and hospitals scope their sequencing projects.

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