[BioC] Limma

[Wei Shi]

Hi Seraya:

	Genes which are present in one condition but not in the other should NOT be removed from your analysis. Only those gene which are absent in both conditions should be filtered out to improve the power to detect differentially expressed genes. It is very likely that a lot of genes of biological interest were not included in your analysis results due to the removal of genes which are present in one condition but not in the other. Have a look at the case study for processing Illumina BeadChip data in limma user guide about the probe filtering.

	Hope this helps.

Cheers,
Wei

On Apr 6, 2011, at 2:05 AM, Seraya Maouche wrote:

> Dear Prof Gordon, dear Bioconductor members,
> 
> I have performed gene expression analysis using Limma (Illumina human
> ref8) comparing two types of cells (referred below as cond1 and cond2).
> Based on detection call, I filtered out transcripts which are absent in
> both types of cells. Transcripts which were expressed only in one cell
> type were included in the analysis.
> 
> I have received the comment below from a reviewer who seems not agree to
> calculate fold change for genes expressed only in one condition. Would
> it be possible to have your opinion about this.
> 
> Thank you in advance for your time,
> S Maouche
> 
> "There is a little conceptual difficulty related to the cond1/cond2
> comparisons for genes that are considered not detected. If a gene
> product is absent (0) in one cell then no fold change can be computed
> (table 2). I don’t know how to circumvent this difficult except by
> saying that the “noise” is considered to reflect low expression. The
> terms “not detected” and “not expressed” are often used interchangeably
> while this is not the same. Detection is based on the definition adopted
> and in many places of the manuscript it should be used in place of
> expression."
> 
> 
> 
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