Clarification of edgeR logFC

[mlkerber]

Hi everyone,
I'm completely new to RNAseq and was hoping someone could help me understand the data output by edgeR. Specifically, could someone explain what the logFC number actually means? I understand that a bigger number means a larger difference in the expression of a gene, but is it literally the log of the fold change?

For example, does a logFC of 2 actually mean a fold change of 10^2? In that case, does a logFC of -2 mean a fold change of 10^-2 or -10^2?

I've been assuming that large, negative logFC numbers meant a large downregulation of that gene in our test sample vs control, but when this interpretation of logFC occurred to me, I became very worried that the expression of these genes was only very slightly different!

Any help would be appreciated.

[Simon Anders]

Usually, log2 fold changes are given, i.e., -2 means a reduction 2^-2, i.e., to a quarter.

[mlkerber]

Thank you! That seems to agree with the documentation that I found for edgeR.