how to interpret log fold change (log2FC) on two cases

just quick bioinformatic question if possible. I did read few papers and could not understand what this log fold change means. to make it simple I have a log2 fold change(log2FC) value 2 between condition A and B. Does it mean A is two times higher compared to B or A is two times smaller compared to B?

Thank you.

Solution 1:

Let's say that for gene expression the logFC of B relative to A is 2. If log2(FC) = 2, the real increase of gene expression from A to B is 4 (2^2) (FC = 4). In other words, A has gene expression four times lower than B, which means at the same time that B has gene expression 4 times higher than A.