Question: RNA-seq z-score normalization prior to clustering
gravatar for acorella
4.5 years ago by
United States
acorella30 wrote:


I would like to perform unsupervised hierarchical clustering on some RNA-seq data, but I was told I need to normalize the data by z-score per gene.

My question is: what type of RNAseq data should z-score normalization be performed on? Is it better to do the normalization on RPKM, CPM, log2 CPM, etc?

I typically represent my RNAseq data as mean-centered log2 CPM: Can I perform z-score normalization per gene on mean-centered log2 CPM? Or is this not advised?


rna-seq normalization • 7.2k views
ADD COMMENTlink modified 4.5 years ago by Ron1.0k • written 4.5 years ago by acorella30

@acorella Can you define the z-socre? is it the z-score normalisation that for each element of a given data as such that e.g. a vector of expression is centered to have mean 0 and scaled to have standard deviation 1? After checking , I came across this post. I believe this is your answer TCGA: What are mRNA expression z-scores? Does TCGA have mRNA expression from controls?

ADD REPLYlink written 4.5 years ago by Mo920

Hi, yes that is the z-score I am referring to, however that post does not answer my question.

My question is, is it appropriate to z-score normalize mean-centered log2 CPM values? Does it matter what type of RNAseq values (CPM, RPKM, TPM, etc) I use to z-score normalize?

ADD REPLYlink modified 4.5 years ago • written 4.5 years ago by acorella30

What is the structure of your data, e.g. #samples, #conditions, #replicates/condition? And are you attempting to find clusters of samples? Or genes? Depending on what you are interested in, using z-scores may not be necessary.

ADD REPLYlink written 4.5 years ago by keith.hughitt280
gravatar for thomas.smith2
4.5 years ago by
United Kingdom
thomas.smith290 wrote:

Hi Acorella,

The Z-score normalisation only really makes sense if the expression values for a given are (approximately) normally distributed. One would expect RPKM to be approximately log normally distributed and CPM to be approximately negative binomially distributed, assuming CPM = counts per million? If you wanted to work from the RPKm or CPM, I'd suggest using the log RPKMs - I'm not sure what to expect from log CPMs?

However, I think you'd be much better off using transcripts per million (TPM) as your unit of expression (see Question: the problem with rpkm (and tpm), and What the FPKM? A review of RNA-Seq expression units). The second link also explains how to convert from RPKM/FPKM to TPM. log(TPM) will be approximately normally distributed and suitable for calculating z-scores.

ADD COMMENTlink written 4.5 years ago by thomas.smith290
gravatar for Ron
4.5 years ago by
United States
Ron1.0k wrote:

Log2CPM can be used to do unsupervised clustering.This should work.

ADD COMMENTlink written 4.5 years ago by Ron1.0k
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