Fold change is often used in analysis of gene expression data in microarray and RNA-Seq experiments, for measuring change in the expression level of a gene.

Fold change is a number describing how much a quantity changes going from an initial to a final value. For example, an initial value of 30 and a final value of 60 corresponds to a fold change of 2 (in common terms, a two-fold increase). A change from 80 to 20 would be a fold change of 0.25, while some practitioners replace a fold-change value that is less than 1 by the negative of its inverse, e.g.0.25 would be a fold change of -4 (in common terms, a four-fold decrease).